单调栈

739. 每日温度

class Solution {
public:
    vector<int> dailyTemperatures(vector<int>& temperatures) {
      stack<int> st;
      vector<int> result(temperatures.size(), 0);
      st.push(0);
      for (int i = 1; i < temperatures.size(); i++) {
        while (!st.empty() && temperatures[i] > temperatures[st.top()]) {
          result[st.top()] = i - st.top();
          st.pop();
        }
        st.push(i);
      }
      return result;
    }
};

496.下一个更大元素 I

class Solution {
public:
    vector<int> nextGreaterElement(vector<int>& nums1, vector<int>& nums2) {
        stack<int> st;
        vector<int> res(nums2.size(), -1);
        st.push(0);
        for (int i = 1; i < nums2.size(); i++) {
            while (!st.empty() && nums2[st.top()] < nums2[i]) {
                res[st.top()] = nums2[i];
                st.pop();
            }
            st.push(i);
        }
        unordered_map<int, int> mp;
        for (int i = 0; i < nums2.size(); i++) {
            mp.insert({nums2[i], i});
        }
        vector<int> ans(nums1.size());
        for (int i = 0; i < nums1.size(); i++) {
            ans[i] = res[mp[nums1[i]]];
        }
        return ans;
    }
};

503.下一个更大元素 II

class Solution {
public:
    vector<int> nextGreaterElements(vector<int>& nums) {
        int n = nums.size();
        vector<int> res(n, -1);
        stack<int> st;
        for (int i = 0; i < n * 2; i++) {
            while (!st.empty() && nums[st.top()] < nums[i % n]) {
                res[st.top()] = nums[i % n];
                st.pop();
            }
            st.push(i % n);
        }
        return res;
    }
};

84. 柱状图中最大的矩形

class Solution {
public:
    int largestRectangleArea(vector<int>& heights) {
        heights.insert(heights.begin(), 0);
        heights.push_back(0);
        stack<int> st;
        st.push(0);
        int ans = 0;
        for (int i = 0; i < heights.size(); i++) {
            while(heights[i] < heights[st.top()]) {
                int mid = st.top();
                st.pop();
                int w = i - st.top() - 1;
                int h = heights[mid];
                ans = max(ans, w * h);
            }
            st.push(i);
        }
        return ans;
    }
};