LeetCode - Refresh - Search a 2D Matrix

Binary search. Just need to convert index.

class Solution {
public:
    bool searchMatrix(vector<vector<int> > &matrix, int target) {
        if (matrix.size() == 0) return false;
        int n = matrix.size(), m = matrix[0].size(), start = 0, end = n*m-1, mid = 0;
        while (start <= end) {
            mid = (start + end)/2;
            if (matrix[mid/m][mid%m] == target) return true;
            if (matrix[mid/m][mid%m] > target) end = mid-1;
            else start = mid+1;
        }
        return false;
    }
};