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：
两次二分搜索，先二分搜索第一列，找出target所在的行，然后二分搜索该行。

public class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        int row = matrix.length;
        if(row == 0) return false;
        int col = matrix[0].length;
        if(col == 0) return false;
        
        if(target < matrix[0][0]) return false;
        int start = 0, end = row -1;
        while(start <= end){
            int mid = (start + end)/2;
            if(matrix[mid][0] ==target)
                return true;
            else if(matrix[mid][0] < target)
                start = mid +1;
            else
                end = mid-1;
        }
        
        int targetRow = end;
        start = 0;
        end = col-1;
        while(start <= end){
             int mid = (start + end)/2;
            if(matrix[targetRow][mid] ==target)
                return true;
            else if(matrix[targetRow][mid] < target)
                start = mid +1;
            else
                end = mid-1;
        }
        return false;
    }
}