2018.10.21 codeforces1071B. Minimum path(dp+욕심+bfs)

#include
using namespace std;
int n,k,mx[2005][2005],maxA;
char mp[2005][2005];
bool vis[2005][2005];
queue<pair<int,int> >q,s;
inline void dfs(int x,int y,int dep){
	if(~mx[x][y])return;
	if(x==1&&y==1)mx[x][y]=(mp[x][y]=='a');
	else{
		if(x^1)dfs(x-1,y,dep-1);
		if(y^1)dfs(x,y-1,dep-1);
		mx[x][y]=(mp[x][y]=='a')+max(mx[x-1][y],mx[x][y-1]);
	}
	if(dep<maxA)return;
	if(mx[x][y]>=dep-k){
		if(dep==maxA)q.push(make_pair(x,y));
		else if(dep>maxA){
			while(!q.empty())q.pop();
			q.push(make_pair(x,y)),maxA=dep;
		}
	}
}
int main(){
	memset(mx,-1,sizeof(mx)),scanf("%d%d",&n,&k);
	for(int i=1;i<=n;++i)scanf("%s",mp[i]+1);
	maxA=k;
	dfs(n,n,n*2-1);
	int sig=26;
	string tmp;
	for(int i=1;i<=min(maxA,n*2-1);++i)tmp=tmp+'a';
	if(maxA>=n*2-1)return cout<<tmp,0;
	if(!maxA)q.push(make_pair(1,1)),tmp=tmp+mp[1][1];
	for(int i=maxA?maxA+1:2;i<=n*2-1;++i){
		sig=26;
		while(!q.empty()){
			pair<int,int>t=q.front();
			q.pop();
			int x=t.first,y=t.second,i,j;
			i=x+1,j=y;
			if(i<=n&&!vis[i][j]){	
				vis[i][j]=1;
				if(mp[i][j]-'a'==sig)s.push(make_pair(i,j));
				else if(mp[i][j]-'a'<sig){
					while(!s.empty())s.pop();
					s.push(make_pair(i,j)),sig=mp[i][j]-'a';
				}
			}
			i=x,j=y+1;
			if(j<=n&&!vis[i][j]){
				vis[i][j]=1;
				if(mp[i][j]-'a'==sig)s.push(make_pair(i,j));
				else if(mp[i][j]-'a'<sig){
					while(!s.empty())s.pop();
					s.push(make_pair(i,j)),sig=mp[i][j]-'a';
				}
			}
		}
		tmp=tmp+(char)(sig+'a');
		while(!s.empty())q.push(s.front()),s.pop();
	}
	cout<<tmp;
	return 0;
}