10.24 So many prefix?

题目：

解题思路：

KMP + DP
$f[i]=f[next[i]]+(i\%2==0)$

Accepted code:

#include<cmath>
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>

using namespace std;

const int M = 200010;
char s[M];
int nxt[M], f[M], n, k, ans;

int main() 
{
  	scanf("%s", s+1); n = strlen(s+1);
  	for(int i = 2; i <= n; ++i) {
    	while(k && s[k+1] != s[i]) k = nxt[k];
    	if(s[k+1] == s[i]) nxt[i] = ++k;
  	}
  	for(int i = 2; i <= n; ++i) {
    	if(i%2 == 0) f[i]++;
    	f[i] += f[nxt[i]];
    	ans += f[i];
  	}
  	printf("%d\n", ans);
  	return 0;
}