Search a 2D Matrix [Leetcode]

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.

Solution: consider the matrix as a one-dimensional array, and then do binary search.

class Solution {
public:
    bool searchMatrix(vector<vector<int> > &matrix, int target) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(matrix.empty() || matrix[0].empty()) return false;
        int cols = matrix[0].size();
        int start = 0, end = matrix.size()*cols-1;
        while(start<=end){
            int middle = (start+end)/2;
            if(matrix[middle/cols][middle%cols]==target)
                return true;
            if(matrix[middle/cols][middle%cols]<target)
                start = middle+1;
            else
                end = middle-1;
        }
        return false;
    }
};