单调栈02

42. 接雨水

class Solution {
public:
    int trap(vector<int>& height) {
        int n = height.size();
        stack<int> st;
        st.push(0);
        int answer = 0;
        for(int i = 1; i < n; i++)
        {
            
            while(!st.empty() && height[st.top()] < height[i])
            {
                int mid = height[st.top()];
                st.pop();
                if(!st.empty())
                    answer += (min(height[i], height[st.top()]) - mid) * (i - st.top() - 1);
            }
            st.push(i);
        }
        return answer;
    }
};

84.柱状图中最大的矩形

class Solution {
public:
    int largestRectangleArea(vector<int>& heights) {
        int answer = 0;
        heights.insert(heights.begin(), 0); // 数组头部加入元素0
        heights.push_back(0); // 数组尾部加入元素0
        stack<int> st;
        st.push(0);
        for(int i = 0; i < heights.size(); i++)
        {

            while(!st.empty() && heights[i] < heights[st.top()])
            {
                int mid = st.top();
                st.pop();
                if(!st.empty())
                    answer = max(answer, heights[mid] * (i - st.top() - 1));
            }
            if(!st.empty() && heights[i] == heights[st.top()])
                st.pop();
            st.push(i);
        }

        return answer;
    }
};