BZOJ 1511: [POI2006]OKR-Periods of Words KMP

#include <bits/stdc++.h>   
#define ll long long 
#define N 1000020 
#define setIO(s) freopen(s".in","r",stdin) 
using namespace std; 
int n,nxt[N];  
char a[N]; 
int main() 
{
    // setIO("input");              
    scanf("%d%s",&n,a+1);   
    ll cnt=0;   
    for(int i=2,j=0;i<=n;++i) 
    {
        for(;j&&a[j+1]!=a[i];)j=nxt[j]; 
        if(a[j+1]==a[i]) ++j;      
        nxt[i]=j;  
    }
    for(int i=2,j=2;i<=n;++i,j=i) 
    { 
        while(nxt[j]) j=nxt[j];   
        if(nxt[i]) nxt[i]=j;        
        cnt+=i-j;  
    }
    printf("%lld\n",cnt); 
    return 0;   
}