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.

两边逼近

class Solution {
public:
    bool searchMatrix(vector<vector<int> > &matrix, int target) {
        int i = 0, j = matrix[0].size()-1;
        while(i <  matrix.size() && j >= 0){
          if(matrix[i][j] == target)
            return true;
          else if(matrix[i][j] < target)
            i++;
          else
            j--;
        }
        return false;
    }
};

Search a 2D Matrix