leetcode 378/ lintcode 401排序

为了可快速解决问题用二分法，这一部分没得可说，用套路把二分法结构搭起来，值得一提的是如何快速算出int helper(vector<vector<int>>& matrix, int value) :算出<=value的个数

• code C++:

//my 从右上开始 best
int helper(vector<vector<int>>& matrix, int value) {
    int i = 0, j = matrix[0].size() - 1;
    int ans = 0;
    while (j >= 0 && i < matrix.size()) {
        if (matrix[i][j] <= value) {
            ans += j + 1;
            i++;
        }
        else
            j--;
    }
    return ans;
}
int kthSmallest(vector<vector<int>>& matrix, int k) {
    int left = matrix[0][0], right = matrix.back().back();
    while (left < right) {
        int mid = left + (right - left) / 2;
        int cnt = helper(matrix, mid);
        if (cnt < k) {
            left = mid + 1;
        }
        else
            right = mid;
    }
    return right;
}