树的同构（HYSBZ - 4337，树的哈希）

一.题目链接：

HYSBZ-4337

二.题目大意：

输出 m 棵树的同构运算等价类.

三.分析：

貌似是个模板题...

原来这就叫做树的哈希，感觉好水啊...

详见代码.

四.代码实现：

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

typedef unsigned long long ull;

const int M = (int)50;
const ull Base = 1331;

int cnt;
struct node
{
    int v, nx;
}Edge[M * 2 + 5];
int head[M + 5];
int sz[M + 5];
ull Hash[M + 5][M + 5];

void init()
{
    cnt = 0;
    memset(head, -1, sizeof(head));
}

void add(int u, int v)
{
    Edge[cnt].v = v;
    Edge[cnt].nx = head[u];
    head[u] = cnt++;
}

ull dfs(int u, int fa)
{
    sz[u] = 1;
    vector <ull> p;
    for(int i = head[u]; ~i; i = Edge[i].nx)
    {
        int v = Edge[i].v;
        if(v == fa) continue;
        p.push_back(dfs(v, u));
        sz[u] += sz[v];
    }
    sort(p.begin(), p.end());
    ull ans = 0;
    int len = p.size();
    for(int i = 0; i < len; ++i)
        ans = ans * Base + p[i];
    return (ans + 1) * sz[u];
}

int main()
{
    int m;
    scanf("%d", &m);
    for(int i = 1; i <= m; ++i)
    {
        int n;
        scanf("%d", &n);
        init();
        int x;
        for(int j = 1; j <= n; ++j)
        {
            scanf("%d", &x);
            if(x == 0)  continue;
            add(j, x);
            add(x, j);
        }
        for(int j = 1; j <= n; ++j) Hash[i][j] = dfs(j, 0);
        sort(Hash[i] + 1, Hash[i] + n + 1);
        for(int j = 1; j <= i; ++j)
        {
            int k = 1;
            while(k <= n && Hash[i][k] == Hash[j][k]) ++k;
            if(k == n + 1)
            {
                printf("%d\n", j);
                break;
            }
        }
    }
    return 0;
}