HDU - 4763 Theme Section （KMP的next数组的应用）

#include <bits/stdc++.h>
using namespace std;
#define maxn 1000010

int Next[maxn];


void GetNext(char s[], int len)
{
        Next[0] = -1;
        int j = -1;
        for (int i = 1; i < len; i++)
        {
                while(j > -1 && s[j+1] != s[i])
                        j = Next[j];
                if (s[j+1] == s[i]) j++;
                Next[i] = j;
        }
}

int main()
{
        int n;
        scanf("%d", &n);
        for (int ca = 1; ca <= n; ca++)
        {
                char s[maxn];
                scanf("%s", s);
                int len = strlen(s), ans = 0;
                GetNext(s, len);//求出Next数组

                for (int i = len-1; i >= 0; i--)
                {
                        int j = i;
                        while(j > -1)
                        {
                                if ((j+1)*2 <= i+1 && len - (j+1) > i)//判断相互的位置的合法性
                                {
                                        int flag = 1;
                                        for (int k = 0; k < j+1; k++)
                                                if (s[len - (j+1) + k] != s[k])
                                                {
                                                        flag = 0;
                                                        break;
                                                }
                                        if (flag) //前缀和后缀匹配
                                                ans = max(ans, j+1);
                                }
                                j = Next[j]; //判断比它小的是否符合
                        }
                }
                printf("%d\n", ans);
        }

}