leetcode-Sliding Window Maximum

Total Accepted: 14811 Total Submissions: 62176 Difficulty: Hard

Given an array nums, there is a sliding window of size k which is moving from the very left of the array to the very right. You can only see thek numbers in the window. Each time the sliding window moves right by one position.

For example,
Given nums = [1,3,-1,-3,5,3,6,7], and k = 3.

Window position                Max---------------               -----[1  3  -1] -3  5  3  6  7       3 1 [3  -1  -3] 5  3  6  7       3 1  3 [-1  -3  5] 3  6  7       5 1  3  -1 [-3  5  3] 6  7       5 1  3  -1  -3 [5  3  6] 7       6 1  3  -1  -3  5 [3  6  7]      7

Therefore, return the max sliding window as [3,3,5,5,6,7].

Note:
You may assume k is always valid, ie: 1 ≤ k ≤ input array's size for non-empty array.

Follow up:
Could you solve it in linear time?

class Solution {public:    vector<int> maxSlidingWindow(vector<int>& nums, int k) {        if(nums.empty()||k<=0)            return vector<int>();        deque<int> qmax;        for(int i=0;i<k;++i){            while(!qmax.empty()&&nums[qmax.back()]<=nums[i])                qmax.pop_back();            qmax.push_back(i);        }            vector<int> res;        res.push_back(nums[qmax.front()]);        int len=nums.size();        int left=1;        for(int right=k;right<len;++left,++right){            while(!qmax.empty()&&nums[qmax.back()]<=nums[right])                qmax.pop_back();            qmax.push_back(right);            if(qmax.front()<left)                qmax.pop_front();            res.push_back(nums[qmax.front()]);            }        return res;    }};