LeetCode 962. 最大宽度坡（单调栈）

示例 1：
输入：[6,0,8,2,1,5]
输出：4
解释：
最大宽度的坡为 (i, j) = (1, 5): A[1] = 0 且 A[5] = 5.

示例 2：
输入：[9,8,1,0,1,9,4,0,4,1]
输出：7
解释：
最大宽度的坡为 (i, j) = (2, 9): A[2] = 1 且 A[9] = 1.
 
提示：
2 <= A.length <= 50000
0 <= A[i] <= 50000

[9,8,1,0,1,9,4,0,4,1]

class Solution {	//错误解
public:
    int maxWidthRamp(vector<int>& A) {
        deque<int> q;
        int maxlen = 0;
        for(int i = 0; i < A.size(); ++i)
        {
        	while(!q.empty() && A[q.back()] > A[i])
        		q.pop_back();
        	if(!q.empty())
        		maxlen = max(maxlen, i-q.front());
        	q.push_back(i);
        }
        return maxlen;
    }
};

class Solution {
public:
    int maxWidthRamp(vector<int>& A) {
        stack<int> s;
        int maxlen = 0, i;
        for(i = 0; i < A.size(); ++i)
        {
        	if(s.empty() || A[s.top()] > A[i])//单调递减栈
                s.push(i);
        }
        for(i = A.size()-1; i >= 0; --i)
        {
            while(!s.empty() && A[i] >= A[s.top()])
            {
                maxlen = max(maxlen, i-s.top());
                s.pop();
            }
        }
        return maxlen;
    }
};