求助，站外题！！！

#include <bits/stdc++.h>
using namespace std;
const int N=1002;
string f[N][N];
char a[N][N];
int kk[N][N];
int main(){
	freopen("treasure.in","r",stdin);
	freopen("treasure.out","w",stdout);
	int n,k;
	std::cin >>n>>k;
	for(int i=1;i<=n;i++)
		for(int j=1;j<=n;j++)
			std::cin >>a[i][j];
	if(a[1][1]!='a'&&k>0)f[1][1]="a",k--;
	else f[1][1]=a[1][1];
	kk[1][1]=k;
	for(int i=1;i<=n;i++){
		for(int j=1;j<=n;j++){
			if(i==1&&j==1)continue;
			if(f[i][j-1]>f[i-1][j]){
				f[i][j]=f[i-1][j];
				kk[i][j]=kk[i-1][j];
			}
			else if(f[i][j-1]<f[i-1][j]){
				f[i][j]=f[i][j-1];
				kk[i][j]=kk[i][j-1];
			}
			else {
				f[i][j]=f[i][j-1];
				kk[i][j]=max(kk[i][j-1],kk[i-1][j]);
			}
			if(a[i][j]!='a'&&kk[i][j]>0)f[i][j]+='a',kk[i][j]--;
			else f[i][j]+=a[i][j];
		}
	}
	std::cout <<f[n][n];
	return 0;
}