kth-smallest-element-in-a-sorted-matrix

//有很多讨论，比如
// https://discuss.leetcode.com/topic/52865/my-solution-using-binary-search-in-c
// https://discuss.leetcode.com/topic/52912/binary-search-heap-and-sorting-comparison-with-concise-code-and-1-liners-python-72-ms

class Solution {
public:
    int kthSmallest(vector<vector<int>>& matrix, int k) {
        // 注意以下算法是直接按照整数范围>>1来进行二分查找的
        int row = matrix.size(), col = matrix[0].size();
        int small = matrix[0][0], big = matrix[row-1][col-1];
        int mid;
        while (small < big) {
            mid = (small + big) >> 1;
            int len = 0;
            for (int i=0; i<row; ++i) {
                len += upper_bound(matrix[i].begin(), matrix[i].end(), mid) - matrix[i].begin();
            }
            if (len < k) {
                small = mid + 1;
            }
            else {
                big = mid;
            }
        }
        return small;
    }
};