[栈] leetcode 503 Next Greater Element II

problem:https://leetcode.com/problems/next-greater-element-ii/

        一道比较简单的单调队列题目。不过由于题目要求是循环的，需要两个pass，第二个pass处理循环生效的next greater，同时需要把下标已经超出范围的队首数据及时pop出来。

class Solution {
public:
    vector<int> nextGreaterElements(vector<int>& nums) 
    {
        deque<int> q;
        vector<int> res(nums.size(), -1);
        for (int i = 0; i < nums.size(); i++) 
        {
            while (!q.empty() && nums[i] > nums[q.back()]) 
            {
                res[q.back()] = nums[i];
                q.pop_back();
            }
            q.push_back(i);
        }
        for (int i = 0; i < nums.size(); i++) 
        {
            while (!q.empty() && nums[i] > nums[q.back()]) 
            {
                res[q.back()] = nums[i];
                q.pop_back();
            }
            while (!q.empty() && q.front() <= i)q.pop_front();
            if (q.empty())break;
        }
        return res;
    }
};