Search a 2D Matrix II

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:

[  [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.

思路：从左下角开始search，target < value, 往上移动，target > value, 往右移动。

public class Solution {    public boolean searchMatrix(int[][] matrix, int target) {        if(matrix == null || matrix.length == 0) return false;        int m = matrix.length; int n = matrix[0].length;        int x = m-1; int y = 0;        while(x >=0 && y<n) {            if(target < matrix[x][y]){                x--;            } else if(target > matrix[x][y]) {                y++;            } else {                return true;            }        }        return false;    }}