汕头市队赛 SRM13 T3

#include<cstdio>
#include<iostream>
#include<cstring>
#include<algorithm>
#include<string>
#define LL long long
using namespace std;
const int M=1e7+7;
int n,k,cnt;
char s[M],q[M];
int main()
{
    freopen("del.in","r",stdin);
    freopen("del.out","w",stdout);
    scanf("%s",s+1);
    n=strlen(s+1); scanf("%d",&k);
    int v=n-k;
    q[++cnt]=s[1];
    for(int i=2;i<=n;i++){
        while(cnt&&q[cnt]>s[i]&&v) cnt--,v--;
        q[++cnt]=s[i];
    }
    for(int i=1;i<=k;i++) printf("%c",q[i]); printf("\n");
    return 0;
}

#include<cstdio>
#include<cstring>
const int M=1000007;
int n,k,c[M][26],deep[M];
char s[M];
void maxs(int& x,int y){if(x<y) x=y;}
int main(){
    freopen("del.in","r",stdin);
    freopen("del.out","w",stdout);
    scanf("%s",s+1); 
    n=strlen(s+1);
    scanf("%d",&k);
    for(int i=1;i<=n;i++){
        int now=s[i]-‘a‘; 
        for(int j=i;j&&!c[j][now];c[j--][now]=i+1);
    }
    for(int i=n;i>=1;i--){
        for(int j=0;j<26;j++){
            int v=c[i][j];
            if(v) maxs(deep[i],deep[v]+1);
        }
    }
    int now=1;
    for(int l=k;l;l--){
        for(int j=0;j<26;j++){
            int v=c[now][j];
            if(v&&deep[v]>=l-1){
                now=v;
                putchar(‘a‘+j);
                break;
            }
        }
    }
    return 0;
}