[USACO Dec06]产奶的模式

---

题目链接 , 求至少重复K次的最长可重叠子串

---

首先二分答案，然后判断是否存在至少 $k$ 个子串 $hash$ 值相等

时间复杂度: $O(N*(logN)^2)$

毕竟 USACO 良心数据，居然不卡自然溢。。。

---

// cogs 1690

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <ctime>
#include <vector>
#include <utility>
#include <stack>
#include <queue>
#include <iostream>
#include <algorithm>

template<class Num>void read(Num &x)
{
    char c; int flag = 1;
    while((c = getchar()) < '0' || c > '9')
        if(c == '-') flag *= -1;
    x = c - '0';
    while((c = getchar()) >= '0' && c <= '9')
        x = (x<<3) + (x<<1) + (c-'0');
    x *= flag;
    return;
}
template<class Num>void write(Num x)
{
    if(x < 0) putchar('-'), x = -x;
    static char s[20];int sl = 0;
    while(x) s[sl++] = x%10 + '0',x /= 10;
    if(!sl) {putchar('0');return;}
    while(sl) putchar(s[--sl]);
}
typedef unsigned int uint;
typedef unsigned long long ull;
const int maxn = 20007;
const uint Base = 1000007;
const ull Basell = Base + 2;

int n, k, a[maxn];
uint hash[maxn], move[maxn];
ull hashll[maxn], movell[maxn];

void init()
{
    read(n), read(k);

    move[0] = movell[0] = 1;
    for(int i = 1; i <= n; i++)
    {
        read(a[i]);

        move[i] = move[i - 1]*Base;
        hash[i] = hash[i - 1]*Base + a[i];
        movell[i] = movell[i - 1]*Basell;
        hashll[i] = hashll[i - 1]*Basell + a[i];
    }
}
bool check(int l)
{
    static std::pair<uint,ull> val[maxn];
    int tot = n - l + 1;
    for(int i = 0; i + l <= n; i++)
        val[i + 1] = std::make_pair(hash[i + l] - hash[i]*move[l], hashll[i + l] - hashll[i]*movell[l]);
    std::sort(val + 1, val + tot + 1);
    for(int i = 1, j = 1; i <= tot; i = j)
    {
        while(j <= tot && val[j] == val[i]) j++;
        if(j - i >= k) return true;
    }
    return false;
}
int solve()
{
    int l = 1, r = n - 1;

    if(!check(l)) return l - 1;
    if(check(r)) return r;

    while(l + 1 != r)
    {
        int mid = (l + r) >> 1;

        if(check(mid)) l = mid;
        else r = mid;
    }
    return l;
}
int main()
{
#ifndef ONLINE_JUDGE
    freopen("patterns.in","r",stdin);
    freopen("patterns.out","w",stdout);
#endif

    init();

    write(solve()); 


#ifndef ONLINE_JUDGE
    fclose(stdin);
    fclose(stdout);
#endif
    return 0;
}