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.

思路：二分查找，先找到行数再找列数。

代码：

class Solution {
 public:
     bool searchMatrix(vector<vector<int> > &matrix, int target) {
         // Start typing your C/C++ solution below
         // DO NOT write int main() function
         int m = matrix.size();
         int n = matrix[0].size();
         if (matrix[0][0]>target || matrix[m-1][n-1]<target) return false;
         int top = 0;
         int down = m-1;
         int mid;
         while (top <= down) {
             mid = (top+down)/2;
             if (target == matrix[mid][0]) return true;
             if (target > matrix[mid][0]) top = mid+1;
             else down = mid-1;
         }
         int row = down;
         int left = 0;
         int right = n-1;
         while (left <= right) {
             mid = (left+right)/2;
             if (target == matrix[row][mid]) return true;
             if (target > matrix[row][mid]) left = mid+1;
             else right = mid-1;
         }
         return false;
     }
 };