https://www.luogu.com.cn/problem/P3435

从当前字符串末尾的next数组的数开始递归，比如长度是8，next[8]是5，递归到5，next[5]是2，递归到2，假如next[2]是0，则当前字符长度为8的最短匹配长度是2

const int N = 1000000 + 50;
string s = " ";
string ss;
int ne[N];

void solve()
{
    int n;
    cin >> n;
    cin >> ss;
    s += ss;
    for (int i = 2, j = 0; i <= n; i++)
    {
        while(j && s[i] != s[j + 1]) j = ne[j];
        if (s[i] == s[j + 1]) j++;
        ne[i] = j;
    }
    ll ans = 0;
    for (int i = 1; i <= n; i++)
    {
        if (ne[i] != 0)
        {
            int j = ne[i];
            while (ne[j]) j = ne[j];
            if (ne[i]) ne[i] = j;
            ans += i - j;
        }
    }
    cout << ans << '\n';
}