HDU 4763 Theme Section ( KMP next函数应用 )

设串为str, 串长为len。

对整个串求一遍next函数，从串结尾开始顺着next函数往前找<=len/3的最长串，假设串长为ans，由于next的性质，所以找到的串肯定满足E……E这种形式，然后就是在str[ans]-str[len-2*ans]中查找是不是包含E，找到就输出，找不到就沿着next向下寻找，缩短串长。

#include <cstdio>

#include <cstring>

#include <cstdlib>

#include <algorithm>



using namespace std;



const int MAXN = 1000100;



char str[MAXN];

char tmp[MAXN];

int nextval[MAXN];

int len;



void GetNextVal( char *s, int lenth )

{

    int i = 0, j = -1;

    nextval[0] = -1;

    while ( i < lenth )

    {

        if ( j == -1 || s[i] == s[j] )

        {

            ++i, ++j;

            if ( s[i] != s[j] ) nextval[i] = j;

            else nextval[i] = nextval[j];

        }

        else j = nextval[j];

    }

    return;

}



bool KMP( char *t, char *s, int lenth, int lenn )

{

    //GetNextVal( t, lenth );

    int i = 0, j = 0;

    while ( j < lenn )

    {

        if ( i == -1 || s[j] == t[i] )

        {

            ++i, ++j;

            if ( i == lenth ) return true;

        }

        else i = nextval[i];

        //if ( i == lenth ) return true;

    }

    return false;

}



int main()

{

    int T;

    scanf( "%d", &T );

    while ( T-- )

    {

        scanf( "%s", str );

        len = strlen(str);

        GetNextVal( str, len );

        int ans = nextval[len];

        while ( ans > len/3 ) ans = nextval[ans];

        while ( ans > 0 )

        {

            if ( KMP( str, &str[ans], ans, len - ans - ans ) )

                break;

            ans = nextval[ans];

        }

        if ( ans < 0 ) printf("%d\n", len/3);

        else printf( "%d\n", ans );

    }

    return 0;

}