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;    
}