HDU 1325(并查集判断一个图是否是一棵树)

梦里梦外; 2022-09-20 12:43 125阅读 0赞

题意：每组数据都以0 0结束，-1 -1结束程序。 每组数据中的每两个数字为一小组，前一个数字代表的结点指向后一个数字代表的结点。

#include <iostream>
    #include <cstring>
    using namespace std;
    
    int father[100010];
    
    int find_father(int x)
    {
        while (father[x] != x)
            x = father[x];
        return x;
    }
    
    bool is_father(int s, int f)
    {
        while (father[s] != f)
        {
            if (father[s] == s) return false;
            s = father[s];
        }
        return true;
    }
    
    int main()
    {
        int cases = 0;
        int max_node, root, f, s;
        bool ok;
        while (cin >> f >> s, f >= 0 && s >= 0)
        {
            ok = true;
            memset(father, 0, sizeof(father));
            root = -1;
            max_node = 0;
            while (f || s)
            {
                if (max_node < f) max_node = f;
                if (max_node < s) max_node = s;
                if (father[f] == 0) father[f] = f;
                if (father[s] == 0) father[s] = s;
                if (father[s] != s) ok = false;
                else if (!is_father(f, s)) father[s] = f;
                else ok = false;
                cin >> f >> s;
            }
            cases++;
            cout << "Case " << cases << " is ";
            if (ok)
                for (int i = 1; i <= max_node; ++i)
                {
                    if (father[i] == 0) continue;
                    int t = find_father(i);
                    if (t != root)
                    {
                        if (root == -1)
                            root = t;
                        else 
                        {
                            ok = false;
                            break;
                        }
                    }
                }
            if (!ok) cout << "not a tree." << endl;
            else cout << "a tree." <<endl;
        }
    }