240. Search a 2D Matrix II（二分查找）

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.

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.

1、坐下角开始找。

2、也可以每行都用二分查找。

 1 class Solution {
 2 public:
 3     bool searchMatrix(vector<vector<int>>& matrix, int target) {
 4         int row = matrix.size() - 1;
 5         if (row+1<=0) return false;
 6         int col = 0;
 7         while(col < int(matrix[0].size()) && row >= 0) {
 8             if(matrix[row][col] > target) {
 9                 row--;
10             } else if (matrix[row][col] < target) {
11                 col++;
12             } else {
13                 return true;
14             }
15         }
16         return false;
17     }
18 };