[LeetCode] Search a 2D Matrix II 搜尋一個二維矩陣之二

Write an efficient algorithm that searches for a value in an m x n matrix. This matrix has the following properties: 

Integers in each row are sorted in ascending from left to right.

Integers in each column are sorted in ascending from top to bottom.

For example,

Consider the following matrix:

Given target = <code>5</code>, return <code>true</code>.

Given target = <code>20</code>, return <code>false</code>.