HYSBZ 2565 最长双回文串 回文树应用

题目链接

题意:求一个最长的由两个回文串拼接来的字串

回文树 顺序 逆序 两边扫描文本串
根据 回文树得到的当前最长后缀回文串 记录以i为结尾的在原串中的最长前缀和最长后缀回文串
维护一个max值即可

回文树是贴的模板
只是有点点理解

#include <stdio.h>#include <iostream>#include <string.h>#include <queue>#define sf scanf#define pf printfusing namespace std;using namespace std;const int MAXN = 100005;struct node {    int next[26];   //边    int len;        //回文串长度    int sufflink;   //后缀指针    int num;};int len;char s[MAXN];node tree[MAXN];int num;            // node 1 - root with len -1, node 2 - root with len 0int suff;           // max suffix palindromelong long ans;bool addLetter(int pos) {    int cur = suff, curlen = 0;    int let = s[pos] - 'a';    while (true) {        curlen = tree[cur].len;                                     //当前可能上一个回文串长度        if (pos - 1 - curlen >= 0 && s[pos - 1 - curlen] == s[pos]) //如果能形成行如XAX的回文串            break;        cur = tree[cur].sufflink;                                   //查找失败 找到更小的回文    }    if (tree[cur].next[let]) {                                      //这个节点已经存在        suff = tree[cur].next[let];                                 //更新最长前缀值        return false;    }    num++;    suff = num;    tree[num].len = tree[cur].len + 2;    tree[cur].next[let] = num;              //加一个节点    if (tree[num].len == 1) {               //只有一个X        tree[num].sufflink = 2;             //指向空串        tree[num].num = 1;        return true;    }    while (true) {        cur = tree[cur].sufflink;        curlen = tree[cur].len;        if (pos - 1 - curlen >= 0 && s[pos - 1 - curlen] == s[pos]) {            tree[num].sufflink = tree[cur].next[let];            break;        }    }    tree[num].num = 1 + tree[tree[num].sufflink].num;    return true;}void initTree() {    num = 2; suff = 2;    tree[1].len = -1; tree[1].sufflink = 1;    tree[2].len = 0; tree[2].sufflink = 1;}void strrev(char *s,int len){    char temp;    for(int i = 0;i < len / 2;++i){        temp = s[i];        s[i] = s[len - i - 1];        s[len - i - 1] = temp;    }}int f[MAXN];int main() {    scanf("%s", s);    len = strlen(s);    initTree();    for (int i = 0; i < len; i++) {        addLetter(i);        f[i] = tree[suff].len;    }    initTree();    strrev(s,len);    int ans = 0;    for(int i = 0;i < len;++i){        addLetter(i);        ans = max(ans,f[len - i - 2] + tree[suff].len);    }    cout << ans << endl;    return 0;}