74. 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.

解题思路为binary search。 先确定在哪一行，然后再确定这一行有没有target。

public class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        if(matrix == null || matrix.length == 0 || matrix[0].length == 0) return false;
        int left = 0, right = matrix.length - 1;
        int mid;
        while(left <= right) {
            mid = left + (right - left) / 2;
            if(matrix[mid][0] == target) {
                return true;
            } else if(matrix[mid][0] < target) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }
        int row = left - 1;
        if(row < 0 || row > matrix.length - 1) return false;
        left = 0;
        right = matrix[0].length - 1;
        while(left <= right) {
            mid = left + (right - left) / 2;
            if(matrix[row][mid] == target) {
                return true;
            } else if(matrix[row][mid] < target) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }
        return false;
    }
}