POJ-3349 Snowflake Snow Snowflakes 哈希

这题直接两个for循环直接TLE，所以这里要先进行一次哈希，然后一次排序，这样只要比较相邻的并且hash值相同的两个串即可。

代码如下：

#include <cstdlib>

#include <cstring>

#include <cstdio>

#include <algorithm>

#define MOD 100000007

using namespace std;



int N, a[6];



struct Node

{

    int a[6], sum;

    bool operator < (Node t) const

    {

        return sum < t.sum;

    }

}num[200010];



void cpto(int a[], int t[], int x)

{

    for (int i = x, j = 0; j < 6; ++j, ++i) {

        t[j] = a[i%6];

    }

}



bool OK(int a, int b)

{

    int t[6], flag, rec[6]; 

    for (int i = 0; i < 6; ++i) { 

    //  依次将这些位放在第一位

        flag = 1;

        cpto(num[a].a, t, i);

        for (int i = 0; i < 6; ++i) {

            if (t[i] != num[b].a[i]) {

                flag = 0;

                break;

            }

        }

        if (flag) {

            return true;

        }

    }

    for (int i = 5, j = 0; i >= 0; --i, ++j) {

        rec[j] = num[a].a[i];

    }

    for (int i = 0; i < 6; ++i) {

        flag = 1;

        cpto(rec, t, i);

        for (int i = 0; i < 6; ++i) {

            if (num[b].a[i] != t[i]) {

                flag = 0;

                break;

            }

        }

        if (flag) {

            return true;

        }

    }

    return false;

}



int main()

{

    int flag;

    while (scanf("%d", &N) == 1) {

        flag = 0;

        for (int i = 0; i < N; i++) {

            scanf("%d %d %d %d %d %d", a, a+1, a+2, a+3, a+4, a+5);    

            memcpy(num[i].a, a, sizeof (a));

            num[i].sum = 0;

            for (int j = 0; j < 6; ++j) {

                num[i].sum ^= a[j];

            }

            num[i].sum %= MOD;

        }

        sort(num, num+N);

        for (int i = 0; i < N && !flag; ++i) {

            for (int j = i + 1; j < N; ++j) {

                if (num[i].sum != num[j].sum) {

                    break;

                }

                if (OK(i, j)) {

                    flag = 1;

                    printf("Twin snowflakes found.\n");

                    break;

                }

            }

        }

        if (!flag) {

            puts("No two snowflakes are alike.");

        }

    }

    return 0;    

}