单调栈
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Last updated
public int trap(int[] height) {
int res = 0;
Stack<Integer> stk = new Stack<>();//存数组的下标索引
int cur = 0; //当前位置的下标
while (cur < height.length) {
//栈不为空 当前位置的值，比栈顶的值（上一个入栈的值，最靠近当前位置的下标索引）要大,入栈
while (!stk.isEmpty() && height[cur] > height[stk.peek()]) {
int m = height[stk.pop()];//记录下这个值，做这一轮计算的底
if (stk.isEmpty()) break;//前探一个位置，没有位置跳出
//计算： 当前位置cur 和 栈顶位置的最小值，组成一个封闭空间，和m这个底相减（木桶原理）, 组成高度
// 下标的相减得到宽度
res += (Math.min(height[cur], height[stk.peek()]) - m) * (cur - stk.peek() - 1);
}
stk.push(cur++);//当前元素比栈顶元素小，入栈，指针后移
}
return res;
}另外一种写法，大同小异:
public int trap(int[] height) {
Stack<Integer> stk = new Stack<>();
int res = 0, cur = 0;
while (cur < height.length) {
if (stk.isEmpty() || height[cur] <= height[stk.peek()]) {
stk.push(cur++);
} else {
//前一个栈弹出的节点
int pre = stk.pop();
if (!stk.isEmpty()) {
//木桶原理，取最小高度
int m = Math.min(height[stk.peek()], height[cur]);
res += (m - height[pre]) * (cur - stk.peek() - 1);
}
}
}
return res;
}public int trap(int[] height) {
//左右索引
int l = 0, r = height.length - 1;
//左右两侧都不能形成一个封闭的区域
//从左侧往右找，一直递增地找
//从右侧往左找，一直递增地找
while (l < r && height[l] <= height[l + 1]) l++;
while (r > l && height[r] <= height[r - 1]) r--;
int res = 0;//结果
while (l < r) {
//左右索引所在的柱子的高度
int left = height[l], right = height[r];
//优先左段
if (left <= right) {
//如果基准的left高度比其右侧的l的高度大，是可以形成雨水的，因为right比left大
//++l精髓，强制向右滑动
while (l < r && left >= height[++l]) {
res += left - height[l];
}
} else {
//如果基准的right高度比其左侧的l的高度大，是可以形成雨水的，因为left比right大
//--r精髓，强制向左滑动
//这里可能会出现相等高度的柱子，体积是0
while (r > l && right >= height[--r]) {
res += right - height[r];
}
}
}
return res;
}另外一种写法，也很巧妙：
public int trap(int[] height) {
int res = 0;
//左右侧的索引
int l = 0, r = height.length - 1;
//l r 对应的height，初始值是MIN
int left = Integer.MIN_VALUE, right = Integer.MIN_VALUE;
while (l < r) {
//获取当前索引 l r的最大高度
left = Math.max(left, height[l]);
right = Math.max(right, height[r]);
//优先低的高度进行计算
if (left < right) {
//l 要强制向右滑动 计算雨水的面积，更新左侧的最大高度left
res += left - height[l++];
left = Math.max(left, height[l]);
} else {
//r 要强制向左滑动 计算雨水的面积，更新右侧的最大高度right
res += right - height[r--];
right = Math.max(right, height[r]);
}
}
return res;
} public int trap(int[] height) {
int n = height.length;
//leftH[i]表示第i个柱子左边最高的柱子的高度
int[] leftH = new int[n];
//rightH[i]表示第i个柱子右边最高的柱子的高度
//上述的两个数组应该是符合单调性的
int[] rightH = new int[n];
//最左边的柱子的左边没有柱子了，leftH[0]=0
for (int i = 0; i < n - 2; i++) {
leftH[i + 1] = Math.max(leftH[i], height[i]);
}
//最右边的柱子的右边没有柱子了，rightH[n-1]=0
for (int i = n - 2; i >= 0; --i) {
rightH[i] = Math.max(rightH[i + 1], height[i + 1]);
}
int res = 0;
//每次取左右两侧的最小值，做高度，每次步进1个长度
for (int i = 1; i < n - 1; i++) {
int m = Math.min(leftH[i], rightH[i]);
if (m > height[i]) {
res += (m - height[i]);
}
}
return res;
}public String removeDuplicateLetters(String s) {
int[] cnt = new int[26];//记录s中每个字母出现的次数
for (char c : s.toCharArray()) cnt[c - 'a']++;
Set<Character> set = new HashSet<>();//记录当前字符是否已经存在在栈内
Deque<Character> stk = new ArrayDeque<>();//单调栈
for (char c : s.toCharArray()) {
if (!set.contains(c)) {//set没有c字母
//1.栈顶字符比当前字符字典序大
//2.栈顶字符在当前字符c后还出现过（栈顶的cnt计数大于1）
while (!stk.isEmpty() && stk.peek() > c && cnt[stk.peek() - 'a'] > 1) {
char t = stk.pop();
cnt[t - 'a']--;
set.remove(t);
}
stk.push(c);
set.add(c);
} else {//set中已经有该字符，也就是在栈内已经出现了1次，后面的同时出现该字符时，可以抛弃
cnt[c - 'a']--;
}
}
StringBuilder sb = new StringBuilder();
while (!stk.isEmpty()) sb.append(stk.pop());
return sb.reverse().toString();
} static class _1st {
public static void main(String[] args) {
_1st handler = new _1st();
int[] nums1 = {3, 4, 6, 5};
int[] nums2 = {9, 1, 2, 5, 8, 3};
int k = 5;
//[9, 8, 6, 5, 3]
handler.maxNumber(nums1, nums2, k);
nums1 = new int[]{6, 7};
nums2 = new int[]{6, 0, 4};
k = 5;
//[6, 7, 6, 0, 4]
handler.maxNumber(nums1, nums2, k);
}
public int[] maxNumber(int[] nums1, int[] nums2, int k) {
int m = nums1.length, n = nums2.length;
int[] res = null;
for (int i = 0; i <= k; i++) {
int[] arr1 = getMaxKArray(nums1, Math.min(i, m));
int[] arr2 = getMaxKArray(nums2, Math.min(k - i, n));
int[] arr = null;
if (arr1.length + arr2.length == k) {
arr = merge(arr1, arr2);
System.out.println(Arrays.toString(arr));
}
if (res == null || arr != null && arr.length == k && compare(arr, 0, res, 0)) {
res = arr;
}
}
return res;
}
//在数组nums中拿到k个数，该数是单调递减的
private int[] getMaxKArray(int[] nums, int k) {
//维护一个单调递减的单调栈
Deque<Integer> stk = new ArrayDeque<>();
for (int i = 0; i < nums.length; i++) {
while (!stk.isEmpty() && stk.peek() < nums[i] && (k - stk.size()) < nums.length - i) {
stk.pop();
}
if (stk.size() < k) stk.push(nums[i]);
}
int[] res = new int[k];
int idx = k - 1;
while (!stk.isEmpty() && idx >= 0) res[idx--] = stk.pop();
return res;
}
//合并A和B数组，生成一个新的数组
private int[] merge(int[] A, int[] B) {
int m = A.length, n = B.length;
int[] res = new int[m + n];
int idx = 0, i = 0, j = 0;
while (i < m || j < n) {
res[idx++] = compare(A, i, B, j) ? A[i++] : B[j++];
}
return res;
}
//比较A和B数组从idx1和idx2分别开始的字典序的大小
private boolean compare(int[] A, int idx1, int[] B, int idx2) {
while (idx1 < A.length && idx2 < B.length && A[idx1] == B[idx2]) {
idx1++;
idx2++;
}
return idx2 == B.length || (idx1 < A.length && A[idx1] > B[idx2]);
}
} public String removeKdigits(String num, int k) {
//维护一个单调递增的单调栈，该栈的大小是len(num)-k的长度
Deque<Character> stk = new ArrayDeque<>();
for (int i = 0; i < num.length(); i++) {
char c = num.charAt(i);
while (!stk.isEmpty() && stk.peek() > c && k > 0) {
stk.pop();
k--;
}
stk.push(c);
}
//防止k没有完全消耗完
/*num="9"
k=1
exprected:"0"
*/
while (k-- > 0) stk.pop();
StringBuilder sb = new StringBuilder();
while (!stk.isEmpty()) sb.append(stk.pop());
boolean headZero = true;//前导零
StringBuilder res = new StringBuilder();
for (char c : sb.reverse().toString().toCharArray()) {
if (c == '0' && headZero) continue;
res.append(c);
headZero = false;
}
return res.toString().equals("") ? "0" : res.toString();
}另
public String removeKdigits(String num, int k) {
Deque<Character> stk = new ArrayDeque<>();
char[] ch = num.toCharArray();
for (int i = 0; i < ch.length; i++) {
while (!stk.isEmpty() && stk.peek() > ch[i] && k > 0) {
stk.pop();
k--;
}
stk.push(ch[i]);
}
while (k-- > 0) stk.pop();
StringBuilder sb = new StringBuilder();
while (!stk.isEmpty()) sb.append(stk.pop());
StringBuilder res = new StringBuilder();
boolean headZero = true;
for (char c : sb.reverse().toString().toCharArray()) {
if (c == '0' && headZero) {
continue;
}
res.append(c);
headZero = false;
}
return res.toString().equals("") ? "0" : res.toString();
}public int largestRectangleArea(int[] heights) {
int n = heights.length;
int[] nums = new int[n + 2];
System.arraycopy(heights, 0, nums, 1, heights.length);
Deque<Integer> stk = new ArrayDeque<>();
int maxArea = 0;
for (int i = 0; i < nums.length; i++) {
//当前的柱子的高度比栈顶的柱子的高度低，计算以弹出的栈顶高度的柱子为高的矩形面积
while (!stk.isEmpty() && nums[stk.peek()] > nums[i]) {
int h = nums[stk.pop()];
maxArea = Math.max(maxArea, h * (i - stk.peek() - 1));
}
stk.push(i);
}
return maxArea;
}另
public int largestRectangleArea(int[] heights) {
Deque<Integer> stk = new ArrayDeque<>();
//设置哨兵
stk.push(-1);
int maxArea = 0;
for (int i = 0; i < heights.length; i++) {
while (stk.peek() != -1 && heights[i] < heights[stk.peek()]) {
int h = heights[stk.pop()];
maxArea = Math.max(maxArea, h * (i - stk.peek() - 1));
}
stk.push(i);
}
//[2,4]case
//如果栈内还存在合法的下标没有出栈，则以数组的右边界为界限计算最大区域
while (stk.peek() != -1) {
int h = heights[stk.pop()];
maxArea = Math.max(maxArea, h * (heights.length - stk.peek() - 1));
}
return maxArea;
}另
public int largestRectangleArea(int[] heights) {
Deque<Integer> stk = new ArrayDeque<>();
//最末尾数设置为0，此前的柱子的高度都会比这个高
int[] nums = new int[heights.length + 1];
for (int i = 0; i < heights.length; i++) nums[i] = heights[i];
int maxArea = 0;
for (int i = 0; i < nums.length; i++) {
while (!stk.isEmpty() && nums[stk.peek()] > nums[i]) {
int h = nums[stk.pop()];
//如果栈空了，就以当前的下标索引为宽度 即 i -0 数组的左边界
maxArea = Math.max(maxArea, h * (stk.isEmpty() ? i : i - stk.peek() - 1));
}
stk.push(i);
}
return maxArea;
}TLE
public int largestRectangleArea(int[] heights) {
int maxArea = 0;
int n = heights.length;
int[] nums = new int[n + 2];
for (int i = 0; i < n; i++) {
nums[i + 1] = heights[i];
}
for (int i = 1; i < nums.length - 1; i++) {
int l = i, r = i;
while (l >= 0 && nums[l] >= nums[i]) l--;
while (r < nums.length && nums[r] >= nums[i]) r++;
maxArea = Math.max(maxArea, nums[i] * (r - l - 1));
}
return maxArea;
}类似1856、907等题的做法
public int largestRectangleArea(int[] heights) {
int n = heights.length;
int[] left = new int[n];
int[] right = new int[n];
// 记录左边第一个小于等于该柱子的下标
left[0] = -1;
for (int i = 1; i < n; i++) {
int t = i - 1;
while (t >= 0 && heights[t] >= heights[i]) t = left[t];
left[i] = t;
}
// 记录每个柱子 右边第一个小于等于该柱子的下标
right[n - 1] = n;
for (int i = n - 2; i >= 0; i--) {
int t = i + 1;
while (t < n && heights[t] >= heights[i]) t = right[t];
right[i] = t;
}
// 计算
int maxArea = 0;
for (int i = 0; i < n; i++) {
int t = heights[i] * (right[i] - left[i] - 1);
maxArea = Math.max(t, maxArea);
}
return maxArea;
} public int[] nextGreaterElement(int[] nums1, int[] nums2) {
int[] res = new int[nums1.length];
Stack<Integer> stk = new Stack<>();
//k:nums2的当前元素，v:当前元素的next greater element
Map<Integer, Integer> map = new HashMap<>();
for (int x : nums2) {
while (!stk.isEmpty() && stk.peek() < x) {
map.put(stk.pop(), x);
}
stk.push(x);
}
//如果当前元素没有NGE，返回-1
for (int i = 0; i < nums1.length; i++) {
res[i] = map.getOrDefault(nums1[i], -1);
}
return res;
}public int[] nextGreaterElements(int[] nums) {
Deque<Integer> stk = new ArrayDeque<>();
int n = nums.length;
int[] res = new int[n];
Arrays.fill(res, -1);
for (int i = 0; i < 2 * n; i++) {
int t = nums[i % n];
while (!stk.isEmpty() && nums[stk.peek()] < t) {
res[stk.pop()] = t;
}
if (i < n) {
stk.push(i);
}
}
return res;
}另，数组模拟栈
public int[] nextGreaterElements(int[] nums) {
int n = nums.length;
int[] ans = new int[n];
Arrays.fill(ans, -1);
// 使用数组模拟栈，bottom 代表栈底，top 代表栈顶
int[] d = new int[n * 2];
int bottom = 0, top = -1;
for (int i = 0; i < n * 2; i++) {
while (bottom <= top && nums[i % n] > nums[d[top]]) {
int u = d[top--];
ans[u] = nums[i % n];
}
d[++top] = i % n;
}
return ans;
}public TreeNode constructMaximumBinaryTree(int[] nums) {
return dfs(nums, 0, nums.length);
}
private TreeNode dfs(int[] nums, int start, int end) {
if (start == end) return null;
int rootIdx = start;
for (int i = start; i < end; i++) {
if (nums[i] > nums[rootIdx]) rootIdx = i;
}
TreeNode root = new TreeNode(nums[rootIdx]);
root.left = dfs(nums, start, rootIdx);
root.right = dfs(nums, rootIdx + 1, end);
return root;
}public int[] dailyTemperatures(int[] T) {
int n = T.length;
int[] res = new int[n];
//存的下标索引
Stack<Integer> stk = new Stack<>();
for (int i = 0; i < n; i++) {
//弹栈 while循环， 如果栈顶的温度比当前的温度小，这个当前的温度是满足题意的首次出现的最高温度
while (!stk.isEmpty() && T[stk.peek()] < T[i]) {
int idx = stk.pop();//之前的那个索引
res[idx] = i - idx;//间隔的天数
}
stk.push(i);//入栈
}
return res;
} public int[] dailyTemperatures(int[] T) {
int n = T.length;
int[] res = new int[n];
//存的下标索引
Deque<Integer> stk = new ArrayDeque<>();
for (int i = 0; i < n; i++) {
//弹栈 while循环， 如果栈顶的温度比当前的温度小，这个当前的温度是满足题意的首次出现的最高温度
while (!stk.isEmpty() && T[stk.peekFirst()] < T[i]) {
int idx = stk.pollFirst();//之前的那个索引
res[idx] = i - idx;//间隔的天数
}
stk.addFirst(i);//入栈
}
return res;
} public int sumSubarrayMins(int[] arr) {
int MOD = (int) 1e9 + 7;
int n = arr.length;
long sum = 0;
Stack<Integer> stk = new Stack<>();
int j, k;
for (int i = 0; i <= n; i++) {
while (!stk.isEmpty() && arr[stk.peek()] > (i == n ? Integer.MIN_VALUE : arr[i])) {
j = stk.pop();
k = stk.isEmpty() ? -1 : stk.peek();
sum += (long) arr[j] * (i - j) * (j - k);
}
stk.push(i);
}
return (int) (sum % MOD);
}另外
public int sumSubarrayMins(int[] arr) {
int MOD = (int) 1e9 + 7;
//存的是当前元素的下标索引
Deque<Integer> stk = new ArrayDeque<>();
int[] A = new int[arr.length + 1];
//最后一个元素作为哨兵
for (int i = 0; i < arr.length; i++) A[i] = arr[i];
// A[A.length-1] = 0;
int N = A.length;
long res = 0;
for (int i = 0; i < N; i++) {
//遍历到的元素比栈顶的元素（遍历到的元素最近的一个元素）要小
while (!stk.isEmpty() && A[i] <= A[stk.peek()]) {
//设置弹出的元素是当前元素
int index = stk.pop();
//前一个元素
int prev = -1;
if (!stk.isEmpty()) prev = stk.peek();
int m = index - prev - 1;//
int n = i - index - 1;
res += (long) (A[index]) * (m + 1) * (n + 1) % MOD;
res %= MOD;
}
stk.push(i);
}
return (int) res;
} public int sumSubarrayMins(int[] nums) {
int MOD = (int) 1e9 + 7;
int n = nums.length;
Deque<Integer> stk = new ArrayDeque<>();
int[] left = new int[n], right = new int[n];
//找到左侧第一个比nums[i]小的下标，维护一个单调递增栈
for (int i = 0; i < n; i++) {
//栈顶元素不小于(即大于等于)当前元素，此时有递减的趋势，弹出栈顶元素
while (!stk.isEmpty() && nums[stk.peek()] >= nums[i]) {
stk.pop();
}
left[i] = stk.isEmpty() ? -1 : stk.peek();
stk.push(i);
}
stk.clear();
//栈顶元素大于(此处没有等于，此处与上一处只能一处有等于，防止重复计算)当前元素，此时有递减的趋势，弹出栈顶元素
for (int i = n - 1; i >= 0; --i) {
while (!stk.isEmpty() && nums[stk.peek()] > nums[i]) {
stk.pop();
}
right[i] = stk.isEmpty() ? n : stk.peek();
stk.push(i);
}
long res = 0;
for (int i = 0; i < n; i++) {
res += 1L * (i - left[i]) * (right[i] - i) * nums[i];
res %= MOD;
}
return (int) res;
} int MOD = (int) 1e9 + 7;
public int sumSubarrayMins(int[] A) {
Stack<Pair> stack = new Stack<>();
int res = 0, t = 0;
for (int i = 0; i < A.length; i++) {
int count = 1;
while (!stack.empty() && stack.peek().val >= A[i]) {
Pair pair = stack.pop();
count += pair.count;
t -= pair.val * pair.count;
}
stack.push(new Pair(A[i], count));
t += A[i] * count;
res += t;
res %= MOD;
}
return res;
}
class Pair {
public int val;
public int count;
public Pair(int val, int count) {
this.val = val;
this.count = count;
}
}public int[] nextLargerNodes(ListNode head) {
List<Integer> list = new ArrayList<>();
while (head != null) {
list.add(head.val);
head = head.next;
}
Stack<Integer> stk = new Stack<>();
int[] res = new int[list.size()];
for (int i = 0; i < list.size(); ++i) {
while (!stk.isEmpty() && list.get(stk.peek()) < list.get(i)) {
int idx = stk.pop();
res[idx] = list.get(i);
}
stk.push(i);
}
return res;
} //AC
public int maxSumMinProduct(int[] nums) {
int MOD = (int) 1e9 + 7;
int n = nums.length;
long[] s = new long[n + 1];
for (int i = 0; i < n; i++) {
s[i + 1] = s[i] + nums[i];
}
int[] left = new int[n], right = new int[n];
Deque<Integer> stk = new ArrayDeque<>();
for (int i = 0; i < n; i++) {
while (!stk.isEmpty() && nums[stk.peek()] > nums[i]) {
stk.pop();
}
left[i] = stk.isEmpty() ? -1 : stk.peek();
stk.push(i);
}
stk.clear();
for (int i = n - 1; i >= 0; --i) {
while (!stk.isEmpty() && nums[stk.peek()] >= nums[i]) {
stk.pop();
}
right[i] = stk.isEmpty() ? n : stk.peek();
stk.push(i);
}
long res = 0;
for (int i = 0; i < n; i++) {
int l = left[i], r = right[i];
res = Math.max(res, 1L * (s[r] - s[l + 1]) * nums[i]);
}
return (int) (res % MOD);
}另，设置哨兵
public int maxSumMinProduct(int[] src) {
int MOD = (int) 1e9 + 7;
//设置前后两个哨兵 0 0
int[] nums = new int[src.length + 2];
for (int i = 0; i < src.length; i++) {
nums[i + 1] = src[i];
}
int n = nums.length;
long[] pre = new long[n + 1];
for (int i = 0; i < n; i++) {
pre[i + 1] = pre[i] + nums[i];
}
Deque<Integer> stk = new ArrayDeque<>();
//右边第一个比它小的元素的下标
int[] right = new int[n];
for (int i = 0; i < n; i++) {
while (!stk.isEmpty() && nums[i] < nums[stk.peek()]) {
right[stk.pop()] = i;
}
stk.push(i);
}
stk.clear();
//左边第一个比它小的元素的下标
int[] left = new int[n];
for (int i = n - 1; i >= 0; --i) {
while (!stk.isEmpty() && nums[i] < nums[stk.peek()]) {
left[stk.pop()] = i;
}
stk.push(i);
}
long res = 0;
for (int i = 0; i < n; i++) {
int l = left[i], r = right[i];
res = Math.max(res, nums[i] * (pre[r] - pre[l + 1]));
}
return (int) (res % MOD);
} public int maxSumMinProduct(int[] nums) {
int MOD = (int) 1e9 + 7;
int n = nums.length;
// left 是严格定义的，left[i] 是左侧最近的严格小于 nums[i] 的元素下标
// right 是非严格定义的，right[i] 是右侧最近的小于等于 nums[i] 的元素下标
int[] left = new int[n], right = new int[n];
Arrays.fill(right, n - 1);
Deque<Integer> stk = new ArrayDeque<>();
for (int i = 0; i < n; i++) {
while (!stk.isEmpty() && nums[stk.peek()] >= nums[i]) {
right[stk.pop()] = i - 1;
}
if (!stk.isEmpty()) {
left[i] = stk.peek() + 1;
}
stk.push(i);
}
long[] pre = new long[n + 1];
for (int i = 0; i < n; i++) {
pre[i + 1] = pre[i] + nums[i];
}
long res = 0;
for (int i = 0; i < n; i++) {
res = Math.max(res, 1L * (pre[right[i] + 1] - pre[left[i]]) * nums[i]);
}
return (int) (res % MOD);
} static class _4th {
public static void main(String[] args) {
_4th handler = new _4th();
String s = "leet";
int k = 3;
char letter = 'e';
int repetition = 1;
// Assert.assertEquals("eet", handler.smallestSubsequence(s, k, letter, repetition));
s = "aaabbbcccddd";
k = 3;
letter = 'b';
repetition = 2;
Assert.assertEquals("abb", handler.smallestSubsequence(s, k, letter, repetition));
}
public String smallestSubsequence(String s, int k, char letter, int repetition) {
//letter这个字符出现的次数
int cnt = 0;
for (char c : s.toCharArray()) {
if (c == letter) cnt++;
}
//m是s中要删除的字符的数量，留下的字符从长度是k个
int n = s.length(), m = n - k;
StringBuilder res = new StringBuilder();
int p = 0;//目前为止letter已扫描了的次数
for (int i = 0; i < n; i++) {
//删除逆序的字母
while (m > 0 && res.length() > 0 && res.charAt(res.length() - 1) > s.charAt(i)) {
if (res.charAt(res.length() - 1) == letter) {
if (repetition > cnt - 1 + p) {//后面的letter不够凑成repetition个letter
break;
}
p--;//删除一个letter
}
res.deleteCharAt(res.length() - 1);
m--;
}
if (s.charAt(i) == letter) {
p++;//扫描letter的次数+1
cnt--;//使用一次letter -1
}
res.append(s.charAt(i));
}
while (res.length() > k) {
if (res.charAt(res.length() - 1) == letter) p--;
res.deleteCharAt(res.length() - 1);
}
for (int i = k - 1; i >= 0; i--) {
if (p < repetition && res.charAt(i) != letter) {
res.setCharAt(i, letter);
p++;
}
}
return res.toString();
}
}思路来着leetcode高赞题解
public long subArrayRanges(int[] nums) {
int n = nums.length;
//计算nums[i]在 nums中 作为最大值和最小值 所出现的最大区间大小
//换言之，需要找到nums[i] 作为最大值，找到左边第一个比nums[i]小的索引j 找到右边第一个比nums[i]小的索引k 区间范围为[k-j]
//同理，需要找到nums[i] 作为最小值，找到左边第一个比nums[i]大的索引j 找到右边第一个比nums[i]大的索引k 区间范围为[k-j]
//这里因为做了两遍的比较，需要特别注意 在 严格相等的时候，只能取一边，另外一边如果重复取，则会重复
long[] maxx = getCnt(nums, false);
long[] minn = getCnt(nums, true);
long res = 0;
//计算当前元素作为最大值和最小值时，出现的次数，计算出「贡献值」
for (int i = 0; i < n; i++) {
res += nums[i] * (maxx[i] - minn[i]);
}
return res;
}
public long[] getCnt(int[] nums, boolean flag) {
int n = nums.length;
int[] left = new int[n], right = new int[n];
Deque<Integer> stk = new ArrayDeque<>();
for (int i = 0; i < n; i++) {
//为true时维持单调递增栈，遇到非严格递减趋势时，弹出栈顶元素「下标」
//这时候找的是左边比小于等于nums[i]的索引，如果左边没有符合条件的下标则设置为-1，即索引0往左的一个位置
while (!stk.isEmpty() && (flag ? nums[stk.peek()] >= nums[i] : nums[stk.peek()] <= nums[i])) {
stk.pop();
}
left[i] = stk.isEmpty() ? -1 : stk.peek();
stk.push(i);
}
stk.clear();
//为true时维持单调递增栈，遇到非严格递减趋势时，弹出栈顶元素「下标」
//这时候找的是右边比小于(此处没有等于)nums[i]的索引，如果左边没有符合条件的下标则设置为n，即索引「len(nums)」往右的一个位置
for (int i = n - 1; i >= 0; i--) {
while (!stk.isEmpty() && (flag ? nums[stk.peek()] > nums[i] : nums[stk.peek()] < nums[i])) {
stk.pop();
}
right[i] = stk.isEmpty() ? n : stk.peek();
stk.push(i);
}
long[] res = new long[n];
//左侧有 (i - left[i]) 和选择，右侧有(right[i] - i)个选择
//根据乘法原理,(i - left[i])*(right[i] - i)
for (int i = 0; i < n; i++) {
res[i] = 1L * (i - left[i]) * (right[i] - i);
}
return res;
}详细参考
