Leetcode240 Search a 2D Matrix II

题目描述

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.

思路

两个指针，一个控制row，一个控制column，因为整个Matrix是有序的，则如果该数比最后一个column（即每一行最大的数）小，则column指针向左移动，如果该数比第一个row大（即每一行最小的数）则row指针向下移动。

代码

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