Leetcode 074 Search a 2D Matrix

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.

思路：先利用每一行的最后一个数与target判断，确定target可能在的行数；再在确定的那一行进行折半查找。

public class S074 {
    public boolean searchMatrix(int[][] matrix, int target) {
        int i  = 0;
        for (;i<matrix.length;i++) {
            if (matrix[i][matrix[0].length-1]>=target) {
                break;
            }
        }
        if (i == matrix.length)
            return false;
        int l = 0, r = matrix[0].length-1;
        //折半查找
        while (l <= r) {
            if (target == matrix[i][(l+r)/2])
                return true;
            else if (target > matrix[i][(l+r)/2])
                l = (l+r)/2+1;
            else 
                r = (l+r)/2-1;
        }
        return false;
    }
}