POJ-1308 Is It A Tree? -并查集

判定是否成环，是否只有一个根，是否为空.

代码如下：

#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <map>
#define MAXN 100000
using namespace std;

int stk[MAXN+5], top, flag;

int set[MAXN+5];

map<int,int>mp;

void init()
{
    for (int i = 1; i <= MAXN; ++i) set[i] = i;
    top = flag = 0;
    mp.clear();    
}

int find(int x)
{
    return set[x] = x == set[x] ? x : find(set[x]);
}

int main()
{
    int x, y, ca = 0;
    init();
    while (scanf("%d %d", &x, &y), x >= 0 && y >= 0) {
        if (!x && !y) {
            int k = 0;
            for (int i = top; i >= 1; --i) {
                if (set[stk[i]] == stk[i]) {
                    ++k;
                }
            }
            if (k == 1 && !flag || top == 0) {
                printf("Case %d is a tree.\n", ++ca);
            }
            else {
                printf("Case %d is not a tree.\n", ++ca);    
            }
            init();
        }
        else {
            if (!mp.count(x)) {
                mp[x] = 1;
                stk[++top] = x;
            }
            if (!mp.count(y)) {
                mp[y] = 1;
                stk[++top] = y;
            }
            int a, b;
            a = find(x), b = find(y);
            if (a != b) {
                set[a] = b;
            }
            else {
                flag = 1;
            }
        }
    }
    return 0;    
}