Codeforces Round #311 (Div. 2) E. Ann and Half-Palindrome (DP+字典树)

题目地址：传送门
先用dp求出所有的符合要求的半回文串，标记出来。然后构造字典树。然后再dfs一遍求出所有节点的子树和，最后搜一遍就能找出第k个来了。
代码如下：

#include <iostream>#include <string.h>#include <math.h>#include <queue>#include <algorithm>#include <stdlib.h>#include <map>#include <set>#include <stdio.h>#include <time.h>using namespace std;#define LL __int64#define pi acos(-1.0)//#pragma comment(linker, "/STACK:1024000000")const int mod=1e9+7;const int INF=0x3f3f3f3f;const double eqs=1e-9;const int MAXN=400000+10;int ok[5001][5001];int k, cnt;char s[6000], str[6000];struct node{        int flag, sum;        node *next[2];};node *newnode(){        int i;        node *p=new node;        for(i=0;i<2;i++){                p->next[i]=NULL;        }        p->flag=0;        p->sum=0;        return p;}void Insert(int st, int len, node *root){        int i, x;        node *p=root;        for(i=st;i<len;i++){                x=s[i]-'a';                if(p->next[x]==NULL)                        p->next[x]=newnode();                p=p->next[x];                if(ok[st][i]) {                        p->flag++;                        p->sum++;                }        }}void dfs(node *u){        for(int i=0;i<2;i++){                if(u->next[i]!=NULL){                        dfs(u->next[i]);                        u->sum+=u->next[i]->sum;                }        }}void seach(node *root){        node *p=root;        while(1){                k-=p->flag;                if(k<=0){                        return ;                }                if(p->next[0]!=NULL){                        if(p->next[0]->sum>=k){                                str[cnt]='a';                                p=p->next[0];                                cnt++;                                continue ;                        }                        else k-=p->next[0]->sum;                }                str[cnt]='b';                p=p->next[1];                cnt++;        }}int main(){        int i, j, len, h;        while(scanf("%s",s)!=EOF){                scanf("%d",&k);                len=strlen(s);                memset(ok,0,sizeof(ok));                for(i=0;i<len;i++){                        ok[i][i]=1;                        if(i<len-1&&s[i]==s[i+1]) ok[i][i+1]=1;                        if(i<len-2&&s[i]==s[i+2]) ok[i][i+2]=1;                        if(i<len-3&&s[i]==s[i+3]) ok[i][i+3]=1;                }                for(i=5;i<=len;i++){                        for(j=0;j<len-i+1;j++){                                h=j+i-1;                                if(s[j]==s[h]&&ok[j+2][h-2]) ok[j][h]=1;                        }                }                node *root;                root=newnode();                for(i=0;i<len;i++){                        Insert(i, len, root);                }                dfs(root);                cnt=0;                seach(root);                for(i=0;i<cnt;i++){                        printf("%c",str[i]);                }                puts("");        }        return 0;}