leetcode 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.

据说是一道很经典的题目，许多大公司面试都有考到，我一开始的思路是从左上角开始比较，如果target小于该值则返回false，若大于则递归查询右边的矩阵和下边的矩阵，但是传上去后出现TLE，想了半天没想到好办法，上网查了一下，思路是这样的：从右上角开始比较，若target大于该值，说明该行不存在target，行数+1，若target小于该值，说明该列不存在target，列数-1，代码如下：

public boolean searchMatrix(int[][] matrix, int target) {    int i=0;    int j=matrix[0].length;    while(i<matrix.length&&j>=0)    {        if(matrix[i][j]==target) return true;        else if(matrix[i][j]<target){            ++i;        }        else if(matrix[i][j]>target){            --j;        }    }    return false;}