从右上角开始搜索，小则下移，大则左移，这个方案真是精妙！
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.
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] ]
Given target = 5, return true.
Given target = 20, return false.
link:{sourcedir}/_0240_SearchA2DMatrixII.java[role=include]