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 the k 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;
    }
};