leetcode 240: Search a 2D Matrix II

Search a 2D Matrix II

Total Accepted: 520 Total Submissions: 1659

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]

这道题是经典题, 我在微软和YELP的onsite和电面的时候都遇到了.

从右上角開始, 比較target 和 matrix[i][j]的值. 假设小于target, 则该行不可能有此数,  所以i++; 假设大于target, 则该列不可能有此数, 所以j--. 遇到边界则表明该矩阵不含target.

[CODE]

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