1380. Lucky Numbers in a Matrix

Description

Given an m x 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]
Explanation: 7 is the only lucky number since it is the minimum in its row and the maximum in its column.

 

Constraints:

• m == mat.length
• n == mat[i].length
• 1 <= n, m <= 50
• 1 <= matrix[i][j] <= 105.
• All elements in the matrix are distinct.

 

Solutions

Solution: Brute Force

• Time complexity: O(Max(mn, m²))
• Space complexity: O(1)

 

JavaScript

/**
 * @param {number[][]} matrix
 * @return {number[]}
 */
const luckyNumbers = function (matrix) {
  const m = matrix.length;
  const n = matrix[0].length;

  const findMinCol = row => {
    let col = 0;

    for (let index = 1; index < n; index++) {
      if (row[index] >= row[col]) continue;
      col = index;
    }
    return col;
  };

  const findMaxColValue = col => {
    let result = 0;

    for (let index = 0; index < m; index++) {
      result = Math.max(matrix[index][col], result);
    }
    return result;
  };

  for (let row = 0; row < m; row++) {
    const col = findMinCol(matrix[row]);
    const current = matrix[row][col];
    const maxValue = findMaxColValue(col);

    if (current === maxValue) return [current];
  }
  return [];
};