240. Search a 2D Matrix II

Write an efficient algorithm that searches for a value in anmxnmatrix. 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:
[
[1,  4,  7, 11, 15],
[2,  5,  8, 12, 19],
[3,  6,  9, 16, 22],
[10, 13, 14, 17, 24],
[18, 21, 23, 26, 30]
]
Giventarget=5, returntrue.
Giventarget=20, returnfalse.

左上角最小右下角最大， 先让raw =0； col = n-1； 如果target在这范围内col--，如果不在row++

代码很简单， leetcode 大神多！