Revise paste Untitled

#include <iostream>
#include <queue>
#include <algorithm>
#include <vector>
#include <algorithm>
#include <set>
#include <cstring>
using namespace std;
int n,m;
int mat[1000][1000];
int dp[1000][1000];
vector<pair<int, int> > v[1000];
int main(){
    ios_base::sync_with_stdio(false);
    cin >> n >> m;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            cin >> mat[i][j];
        }
    }
    for(int i = 0; i < n - 1; i++) {
        for(int j= 0; j < m; j++) {
            v[i].push_back(make_pair(mat[i][j] + mat[i + 1][j], j));
        }
        sort(v[i].begin(), v[i].end());
        reverse(v[i].begin(), v[i].end());
    }
    memset(dp,0,sizeof dp);
    dp[0][0] = mat[0][0];
    for (int i = 0; i < n-1; i++) {
        for (int j = 0; j < m; j++) {
            for (int k = 0; k < min(3, m) ; k++) {
                if(v[i][k].second != j) {
                    dp[i + 1][v[i][k].second] = max(dp[i + 1][v[i][k].second], dp[i][j] + v[i][k].first);
                }
            }
        }
    }
    int res = dp[n-1][m-1];
    for(int i = 0; i < m-1; i++) {
        if(n == 2 && i == 0){
            continue;
        }
        res = max(res,dp[n-1][i] + mat[n-1][m-1]);

}
    cout << res;
}