回文自动机模板

#include <bits/stdc++.h>

const int N = 500010;

struct Edge {
    int nex, v;
}edge[N]; int tp;

int tr[N][26], fail[N], len[N], tot = 1, last = 1, e[N], Ans, stk[N], top, n;
char str[N];

inline void add(int x, int y) {
    edge[++tp].v = y;
    edge[tp].nex = e[x];
    e[x] = tp;
}

inline void init() {
    len[1] = -1;
    fail[0] = fail[1] = 1;
    return;
}

inline int getpos(int x, int i) {
    while(str[i] != str[i - len[x] - 1]) {
        x = fail[x];
    }
    return x;
}

inline void insert(int i, int f) {
    int x = getpos(last, i);
    if(!tr[x][f]) {
        ++tot;
        fail[tot] = tr[getpos(fail[x], i)][f];
        len[tot] = len[x] + 2;
        tr[x][f] = tot;
    }
    last = tr[x][f];
    return;
}

inline void Insert() {
    for(int i = 1; i <= n; i++) {
        insert(i, str[i] - 'a');
    }
    return;
}

void DFS(int x, int p) {
    if(len[x - 1] >= 2 && (!(len[x - 1] & 1))) {
        stk[++top] = len[x - 1];
        while(stk[p] * 2 < stk[top]) {
            ++p;
        }
        if(stk[p] * 2 == stk[top]) {
            Ans = std::max(stk[top], Ans);
        }
    }
    for(int i = e[x]; i; i = edge[i].nex) {
        int y = edge[i].v;
        DFS(y, p);
    }
    if(len[x - 1] >= 2 && (!(len[x - 1] & 1))) {
        top--;
    }
    return;
}

int main() {

    scanf("%d%s", &n, str + 1);
    init();
    Insert();

    add(2, 1);
    for(int i = 2; i <= tot; i++) {
        add(fail[i] + 1, i + 1);
    }

    DFS(2, 0);
    printf("%d
", Ans);
    return 0;
}

以SHOI2011 双倍回文为例。