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1380. Lucky Numbers in a Matrix

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Description

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.

Solutions

Python3

class Solution:
    def luckyNumbers (self, matrix: List[List[int]]) -> List[int]:
        row_min = {min(rows) for rows in matrix}
        col_max = {max(cols) for cols in zip(*matrix)}
        return [e for e in row_min if e in col_max]

Java

class Solution {
    public List<Integer> luckyNumbers (int[][] matrix) {
        int m = matrix.length, n = matrix[0].length;
        Set<Integer> rowMin = new HashSet<>();
        List<Integer> res = new ArrayList<>();
        for (int i = 0; i < m; ++i) {
            int min = Integer.MAX_VALUE;
            for (int j = 0; j < n; ++j) {
                min = Math.min(min, matrix[i][j]);
            }
            rowMin.add(min);
        }

        for (int j = 0; j < n; ++j) {
            int max = Integer.MIN_VALUE;
            for (int i = 0; i < m; ++i) {
                max = Math.max(max, matrix[i][j]);
            }
            if (rowMin.contains(max)) {
                res.add(max);
            }

        }
        return res;
    }
}

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