判断是否是树（Is It A Tree?）

#include <stdio.h>
#define max_num  100000+10
typedef struct
{
    int vis, root, conn;
}Node;
Node node[max_num];
void init()
{
    for(int i = 0; i < max_num; i++)
    {
        node[i].vis = 0;
        node[i].root = i;
        node[i].conn = 0;
    }
}
int find_root(int x)
{
    while(x != node[x].root)
        x = node[x].root;
    return x;
}
void union_set(int x, int y)
{
    x = find_root(x);
    y = find_root(y);
    if(x != y)
        node[y].root = x;
}
int main()
{
    int n, m;
    bool flag = true;
    int case_num = 1;
    init();
    //freopen("input.txt", "r", stdin);
    while(scanf("%d%d", &n, &m), n >= 0 && m >= 0)
    {
        if(!flag && (n != 0 && m != 0))
            continue;
        if(n == 0 && m == 0)
        {
            int root_num = 0;
            for(int i = 1; i < max_num; i++)
            {
                if(find_root(i) == i && node[i].vis)
                    root_num++;
                if(node[i].conn > 1)
                    flag = false;
            }
            if(root_num > 1)
                flag = false;
            if(flag)
                printf("Case %d is a tree.\n", case_num++);
            else
                printf("Case %d is not a tree.\n", case_num++);
            flag = true;
            init();
            continue;
        }
        if(n != m && find_root(n) == find_root(m))
            flag = false;
        else
        {
            node[n].vis = 1;
            node[m].vis = 1;
            node[m].conn++;
            union_set(n, m);
        }
    }
    return 0;
}