leetcode 74. Search two-dimensional matrix (half)

Prepared by an efficient algorithm to determine the mxn matrix, the presence or absence of a target value. This matrix has the following characteristics:

An integer of from left to right in each row in ascending order.
The first integer is greater than the last row of each integer previous row.
Example 1:

Input:
Matrix = [
[. 1,. 3,. 5,. 7],
[10,. 11, 16, 20 is],
[23 is, 30, 34 is, 50]
]
target =. 3
Output: true
Example 2:

Input:
Matrix = [
[. 1,. 3,. 5,. 7],
[10,. 11, 16, 20 is],
[23 is, 30, 34 is, 50]
]
target = 13 is
output: false

Source: stay button (LeetCode)
link: https: //leetcode-cn.com/problems/search-a-2d-matrix

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