正题

ybtoj KMP-4

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题目大意

给出字符串S，问你有多少个子串满足ABA的形式，且$|A|⩾k,|B|⩾1$

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解题思路

可以先枚举左端点，然后跑KMP，使其满足左右两个A，然后判断A,B是否满足条件，然后计算答案

时间复杂度$O(n^2)$，理论上是过不了的，但它就是过了，题解还是这么写的

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代码

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#define ll long long
#define N 15010
using namespace std;
int k, n, ans, nx[N];
char s[N];
void kmp(char* s)
{int n = strlen(s+1); nx[1] = 0;for (int i = 2, j = 0; i <= n; ++i){while (s[i] != s[j + 1] && j) j = nx[j];if (s[i] == s[j + 1]) j++;nx[i] = j;}for (int i = 2, j = 0; i <= n; ++i){while (s[i] != s[j + 1] && j) j = nx[j];if (s[i] == s[j + 1]) j++;if (i - j <= j) j = nx[j];//|B|⩾0if (j >= k) ans++;}return;
}
int main()
{scanf("%s%d", s+1, &k);n = strlen(s+1);for (int i = 0; i < n - k * 2; ++i)kmp(s + i);printf("%d", ans);return 0;
}