【HDOJ】1325 Is It A Tree?

并查集。需要考虑入度。

#include <stdio.h>
#include <string.h>

#define MAXNUM 10005

int bin[MAXNUM];
int degree[MAXNUM];
int nums[MAXNUM];

int find(int x) {
    int r = x;

    while (bin[r] != r)
        r = bin[r];

    return r;
}

int main() {
    int x, y, fx, fy, n, case_n = 0;
    int i, flg;

    while (1) {
        scanf("%d %d", &x, &y);
        if (x<0 && y<0)
            break;
        memset(degree, 0, sizeof(degree));
        n = 0;
        ++case_n;
        if (x==0 && y==0) {
            printf("Case %d is a tree.
", case_n);
            continue;
        }
        for (i=0; i<MAXNUM; ++i)
            bin[i] = i;
        fx = find(x);
        fy = find(y);
        bin[fy] = fx;
        degree[y]++;
        flg = 1;
        nums[n++] = x;
        nums[n++] = y;
        while (1) {
            scanf("%d %d", &x, &y);
            if (x==0 && y==0)
                break;
            fx = find(x);
            fy = find(y);
            if (fx != fy) {
                bin[fy] = fx;
                degree[y]++;
            } else {
                bin[fy] = fx;
                degree[y]++;
                flg = 0;
            }
            nums[n++] = x;
            nums[n++] = y;
        }
        fx = find(nums[0]);
        for (i=1; i<n; ++i) {
            fy = find(nums[i]);
            if (fx != fy) {
                flg = 0;
                break;
            }
        }
        for (i=0; i<n; ++i) {
            if (degree[nums[i]] > 1) {
                flg = 0;
                break;
            }
        }
        if (flg)
            printf("Case %d is a tree.
", case_n);
        else
            printf("Case %d is not a tree.
", case_n);
    }

    return 0;
}