74-Search a 2D Matrix

【题目】

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 from left to right.
• The first integer of each row is greater than the last integer of the previous row.

For example,

Consider the following matrix:

[
  [1,   3,  5,  7],
  [10, 11, 16, 20],
  [23, 30, 34, 50]
]

Given target = 3, return true.

【分析】

1.采用二分搜索的思想

2.假设从右上角开始搜索，若当前值大于target，则向左移；若当前值小于target，则向右移。

【算法实现】

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