Is It A Tree? UVA - 615

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#include <iostream>
#include <cstdio>

using namespace std;

const int MAXN = 100000;

int v[MAXN+1];
bool visited[MAXN+1];
int length = MAXN;
bool nocircleflag;    // 判断是否成环
int edgecount;      // 边的个数

int Find(int x) {
    if(x == v[x])
        return x;
    else
        return v[x] = Find(v[x]);
}

bool Union(int x, int y) {
    edgecount++;
    visited[x] = true;
    visited[y] = true;

    x = Find(x);
    y = Find(y);
    
    if(x == y) {
        nocircleflag = false;
        return false;
    } 
    else {
        v[x] = y;
        return true;
    }
}

bool isconnect() {
    int rootcount = 0;
    for( int i=0 ; i<=MAXN ; i++ )
        if(visited[i])
            rootcount++;
    if(rootcount == edgecount + 1)
        return true;
    return false;
}


void init() {
    nocircleflag = true;
    edgecount = 0;
    for(int i=0; i<=length; i++) {
        v[i] = i;
        visited[i] = false;
    }
}

int main()
{
    int src, dest, caseno=0;

    while(~scanf("%d%d", &src, &dest) && (src != -1 && dest != -1)) {

        init();

        if(src==0 && dest==0) {
            //为空树  
            printf("Case %d is a tree.\n", ++caseno);
        } else {
            Union(src, dest);
            while(1){
                scanf("%d%d", &src, &dest);
                if( src==0 && dest==0 )
                    break;

                Union(src, dest);
            }

            if(nocircleflag && isconnect())
                printf("Case %d is a tree.\n", ++caseno);
            else
                printf("Case %d is not a tree.\n", ++caseno);
        }
    }

    return 0;
}