LeetCode Problem : Search a 2D Matrix

Problem

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.

Code

bool searchMatrix(vector<vector<int> > &matrix, int target) {
    // IMPORTANT: Please reset any member data you declared, as
    // the same Solution instance will be reused for each test case.
    int m = matrix.size();
    int n = matrix[0].size();
    int top = 1,bottom = m,row = -1;
    while(top <= bottom){
        int mid = (top + bottom)/2;
        if(matrix[mid - 1][0] > target)
            bottom = mid - 1;
        else if(matrix[mid - 1][n - 1] < target)
            top = mid + 1;
        else{
            row = mid - 1;
            break;
        }
    }
    if( row == -1)
        return false;
    int l = 1,r = n;
    while(l <= r){
        int mid = (l + r)/2;
        if(matrix[row][mid - 1] > target)
            r = mid - 1;
        else if(matrix[row][mid - 1] < target)
            l = mid + 1;
        else{
            return true;
        }
    }
    return false;
}