马拉车（manacher）算法

char s[N], ms[N];
int p[N];
inline void init(char *s) {
    ms[0] = ‘@‘, ms[1] = ‘#‘;
    for (int i = 1; s[i]; ++i)
        ms[i<<1] = s[i], ms[i<<1|1] = ‘#‘;
    int l = strlen(s+1);
    ms[l*2+2] = ‘^‘, ms[l*2+3] = 0;
}

inline void manacher(char *s) {
    init(s);
    int id = 0, mx = 0;
    int l = strlen(ms);
    for (int i = 1; i < l; ++i) {
        p[i] = mx>i? min(mx-i, p[2*id-i]): 1;
        while(p[i]+i < l && ms[i+p[i]] == ms[i-p[i]]) ++p[i];
        if(p[i] + i > mx) {
            mx = p[i] + i;
            id = i;
        }
    }
}