leetcode378

public class Solution {
    public int KthSmallest(int[,] matrix, int k) {
        var row = matrix.GetLength(0);
            var col = matrix.GetLength(1);

            var list = new List<int>();

            for (int i = 0; i < row; i++)
            {
                for (int j = 0; j < col; j++)
                {
                    list.Add(matrix[i, j]);
                }
            }

            list = list.OrderBy(x => x).ToList();
            return list[k - 1];
    }
}

https://leetcode.com/problems/kth-smallest-element-in-a-sorted-matrix/#/description

补充一个多次使用二分查找的解决方案：

public class Solution {
    public int KthSmallest(int[][] matrix, int k) {
        int low = matrix[0][0];
        int high = matrix[matrix.Length - 1][matrix[0].Length - 1];
        int count = 0;
        while (low <= high)
        {
            int mid = low + (high - low) / 2;
            count = GetCount(matrix, mid);
            if (count < k)
            {
                low = mid + 1;
            }
            else
            {
                high = mid - 1;
            }
        }
        return low;
    }
    
    
    private int GetCount(int[][] matrix, int num)
    {
        int ans = 0;
        int i = 0;
        int j = matrix[0].Length - 1;
        while (i < matrix.Length && j >= 0)
        {
            if (matrix[i][j] > num)
            {
                j--;
            }
            else
            {
                ans += j + 1;
                i++;
            }
        }
        return ans;
    }
}