Given an $m \times n$ matrix of distinct numbers, return the lucky number in the matrix.

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

Constraints:

• $m =$ matrix.length

• $n =$ matrix[i].length

• $1 \leq n$, $m \leq 50$

• $1 \leq$ matrix[i][j] $\leq 10^5$

• All elements in the matrix are distinct.

The core idea behind the solution is to recognize that there can be, at most, one lucky number in the matrix. This is proven by contradiction, as having two such numbers would violate the unique conditions for being a lucky number.