74. Search a 2D Matrix

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.

Example 1:

Input:
matrix = [
  [1,   3,  5,  7],
  [10, 11, 16, 20],
  [23, 30, 34, 50]
]
target = 3
Output: true

Example 2:

Input:
matrix = [
  [1,   3,  5,  7],
  [10, 11, 16, 20],
  [23, 30, 34, 50]
]
target = 13
Output: false

my code:

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        int row = matrix.size();
        if (row == 0) return false;
        int col = matrix[0].size();
        if (col == 0) return false;
        int flag = 0;
        for (int i = 0; i < row; ++i) {
            if (matrix[i][col-1] >= target) {
                flag = i;
                break;
            }    
        }
        
        int l = 0, r = col;
        while (l < r) {
            int m = (l + r) / 2;
            if (matrix[flag][m] == target) return true;
            if (matrix[flag][m] > target) {
                r = m;
            }
            if (matrix[flag][m] < target) {
                l = m+1;
            }
        }
        return false;
    }
};

Runtime: 8 ms, faster than 97.84% of C++ online submissions for Search a 2D Matrix.