hdu 1325 Is It A Tree?

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#include <iostream>#include <cstring>#include <cstdio>using namespace std;const int M = 1005;int vist[M], Set[M];int du[M];int a, b, maxn;int flag, cont, k;/*形成树的条件是， 每一个节点的入度只能为1， 且根只能有一个。*/int Find(int x) {    return x == Set[x]? x : Set[x] = Find(Set[x]);}void Union(int x, int y) {    x = Find(x);    y = Find(y);    if(x != y) {        Set[y] = x;    }}void init() {    for(int i = 1; i < M; i++) {        Set[i] = i;        vist[i] = 0;        du[i] = 0;    }    flag = 1;    cont = 0;    k = 0;    maxn = 0;}int main(){    int q = 1;     init();    while(scanf("%d%d", &a, &b) != EOF) {        if(a < 0 && b < 0)            break;        if(a == 0 && b == 0) {            for(int i = 1; i < M; i++) {                if(vist[i] && Set[i] == i) {                    cont++;                }                if(du[i] > 1) {                    flag = 0;                    break;                }            }            if(cont >1)                 flag = 0;            if(flag)                 printf("Case %d is a tree.\n", q++);            else                printf("Case %d is not a tree.\n", q++);            flag = 0;            init();            continue;        }        if(a != b && Find(a) == Find(b)) {            flag = 0;        }        else {            vist[a] = 1;            vist[b] = 1;            du[b]++;            Union(a, b);        }    }    return 0;}
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