1380. Lucky Numbers in a Matrix (E)

Given a m * n matrix of distinct numbers, return all lucky numbers in the matrix in any order.

A lucky number is an element of the matrix such that it is the minimum element in its row and maximum in its column.

Example 1:

Input: matrix = [[3,7,8],[9,11,13],[15,16,17]]
Output: [15]
Explanation: 15 is the only lucky number since it is the minimum in its row and the maximum in its column

Example 2:

Input: matrix = [[1,10,4,2],[9,3,8,7],[15,16,17,12]]
Output: [12]
Explanation: 12 is the only lucky number since it is the minimum in its row and the maximum in its column.

Example 3:

Input: matrix = [[7,8],[1,2]]
Output: [7]

Constraints:

• m == mat.length
• n == mat[i].length
• 1 <= n, m <= 50
• 1 <= matrix[i][j] <= 10^5.
• All elements in the matrix are distinct.

题意

在矩阵中，如果一个数既是它所在行的最小值，又是它所在列的最大值，则称这个数为幸运数。找到矩阵中所有的幸运数。

思路

每次先找到一行的最小值，再判断它是不是所在列的最大值。

代码实现

class Solution {
    public List<Integer> luckyNumbers (int[][] matrix) {
        List<Integer> ans = new ArrayList<>();
        if (matrix.length == 0) {
            return ans;
        }
        for (int i = 0; i < matrix.length; i++) {
            // 找到行的最小值
            int min = Integer.MAX_VALUE;
            int pos = 0;
            for (int j = 0; j < matrix[0].length; j++) {
                if (matrix[i][j] < min) {
                    min = matrix[i][j];
                    pos = j;
                }
            }
            // 判断是不是列的最大值
            boolean flag = true;
            for (int j = 0; j < matrix.length; j++) {
                if (matrix[j][pos] > min) {
                    flag = false;
                    break;
                }
            }
            if (flag) {
                ans.add(min);
            }
        }
        return ans;
    }
}