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.

Analysis

1. 从左上角开始找很麻烦，要往两个方向找
2. 从右上角开始找，左边的都小于该数，下面的都大于该数，每次排除一行或一列
3. note：也可以从左下角开始找

Code

public class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        int m=matrix.length;
        if(m==0) {return false;}
        int n=matrix[0].length;

        int row = 0;
        int col = n-1;

        while(row<m && col>=0){
            if (target==matrix[row][col]){return true;}
            else if(target>matrix[row][col]){row++;}
            else {col--;}
        }

        return false;

    }
}
//note: use while but not for since we want to control the row++, and col-- based on some condition, not automatically

Reference

Ref1 Ref2