class Solution {
public:
    int largestRectangleArea(vector<int>& heights) {
    heights.push_back(-1);//同理，我们希望栈中所有数据出栈，所以给数组最后添加一个负数
    stack<int> st;
    int ret = 0, top;
    for (int i = 0; i < heights.size(); i++)
    {
        if (st.empty() || heights[st.top()] <= heights[i])
        {
            st.push(i);
        }
        else
        {
            while (!st.empty() && heights[st.top()] > heights[i])
            {
                top = st.top();
                st.pop();
                //i-top指的是当前矩形的宽度，heights[top]就是当前的高度
                //再次强调栈中现在为单调递增
                int tmp = (i - top)*heights[top];
                if (tmp > ret)
                    ret = tmp;
            }
            st.push(top);
            heights[top] = heights[i];
        }
    }
    return ret;
}

};