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: 
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: 
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: 


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.

## Solution: Pre-processing

Two pass. First pass, record the min val of each row, and max val of each column.
Second pass, identify lucky numbers.

Time complexity: O(m * n)
Space complexity: O(m + n)

## C++

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