lintcode Sliding Window Median

class Solution {
public:
    /**
     * @param nums: A list of integers.
     * @return: The median of the element inside the window at each moving
     */
    vector<int> medianSlidingWindow(vector<int> &nums, int k) {
        // write your code here
        map<int, int> rec;
        vector<int> res;
        
        if (k == 0 || nums.empty() || nums.size() < k)
            return res;
            
        int n = nums.size();
        for (int i = 0; i < k; ++i)
            ++rec[nums[i]];
        
        // 虽然不明白怎么回事但是AC了
        // 这部分再想想
        auto getMid = [&](){
            int target = k >> 1;
            target -= 1 - (k & 0x1);
            
            int cur = 0;
            auto iter = rec.begin();
            for ( ; iter != rec.end(); ++iter){
                cur += iter->second;
                if (cur > target) break;
            }
            
            return iter->first;
        };
        
        res.push_back(getMid());
        
        for (int i = k; i < n; ++i){
            auto iter = rec.find(nums[i - k]);
            --iter->second;
            if (iter->second == 0) rec.erase(iter);
            ++rec[nums[i]];
            
            res.push_back(getMid());
        }
        
        return res;
    }
};