https://qqwhale.com/problem/5315

#include <bits/stdc++.h>
#include <iostream>
using namespace std;
int a[1151][1151];
int main(){
	int n,m;
	cin>>n>>m;
    for(int i=1;i<=n*4;i++){
        for(int j=1;j<=m*4;j++){
            a[i][j]=-1;
        }
    }
    for(int i=1;i<=n;i++){
		for(int j=1;j<=m;j++){
           // cout<<i<<" "<<j<<endl;
			cin>>a[i][j];
		}
	}
    int sum=1;
	for(int i=1;i<=m;i++){
		int x=i;
		for(int j=sum;x>=1;x--,j++){            
            //cout<<" "<<x<<" "<<j<<endl;
           if(a[j][x]==-1)continue;
			cout<<a[j][x];
		}
	}
    for(int i=2;i<=n;i++){
        int x=i;
        for(int j=m;x<=n;x++,j--){            
             if(a[x][j]==-1)continue;
            cout<<a[x][j];
        }
    }
	return 0;
}