E「水」悠悠碧波

题目地址：

https://ac.nowcoder.com/acm/contest/6226/E

基本思路：

参考代码：

#pragma GCC optimize(2)
#pragma GCC optimize(3)
#include <bits/stdc++.h>
using namespace std;
#define IO std::ios::sync_with_stdio(false)
#define int long long
#define rep(i, l, r) for (int i = l; i <= r; i++)
#define per(i, l, r) for (int i = l; i >= r; i--)
#define mset(s, _) memset(s, _, sizeof(s))
#define pb push_back
#define pii pair <int, int>
#define mp(a, b) make_pair(a, b)
#define INF (int)1e18

inline int read() {
  int x = 0, neg = 1; char op = getchar();
  while (!isdigit(op)) { if (op == '-') neg = -1; op = getchar(); }
  while (isdigit(op)) { x = 10 * x + op - '0'; op = getchar(); }
  return neg * x;
}
inline void print(int x) {
  if (x < 0) { putchar('-'); x = -x; }
  if (x >= 10) print(x / 10);
  putchar(x % 10 + '0');
}

namespace KMP {
    vector<int> next;

    void build(const string &pattern) {
      int n = pattern.length();
      next.resize(n + 1);
      for (int i = 0, j = next[0] = -1; i < n; next[++i] = ++j) {
        while (~j && pattern[j] != pattern[i]) j = next[j];
      }
    }

    vector<int> match(const string &pattern, const string &text) {
      vector<int> res;
      int n = pattern.length(), m = text.length();
      build(pattern);
      for (int i = 0, j = 0; i < m; ++i) {
        while (j > 0 && text[i] != pattern[j]) j = next[j];
        if (text[i] == pattern[j]) ++j;
        if (j == n) res.push_back(i - n + 1), j = next[j];
      }
      return res;
    }
};

string s,t;
signed main() {
  IO;
  cin >> s;
  KMP::build(s);
  vector<int> nxt = KMP::next;
  int sz =nxt.size();
  int tmp = nxt[sz - 1];
  while (tmp != -1){
    t.clear();
    for(int i = 0 ; i < tmp ; i++) t += s[i];
    vector<int> vec = KMP::match(t,s);
    if(vec.size() >= 3) break;
    tmp = nxt[tmp];
  }
  cout << t << '\n';
  return 0;
}