Codeup Problem D: More is better

原题地址

http://codeup.hustoj.com/problem.php?cid=100000615&pid=3

解题思路

题意就是求集合的最大元素数量。使用并查集的操作即可~

注意事项

1.还是那个老问题，在合并集合的时候如果要更新集合规模，一定要注意书写的顺序~详情见代码。
2.注意特判，n = 0时输出1。

参考代码

#include 
using namespace std;
#define pb push_back
typedef double db;
typedef long long LL;
typedef vector VI;
const int inf = 2e9;
const LL INF = 8e18;
const int maxn = 1e7 + 10;

int father[maxn];

int findFather(int x) {
	if (father[x] < 0) return x;
	else {
		return father[x] = findFather(father[x]);
	}
}

void Union(int a, int b) {
	int fa = findFather(a);
	int fb = findFather(b);
	if (fa != fb) {
		//fa元素比fb多
		if (father[fa] < father[fb]) {
			father[fb] += father[fa]; //一定一定这两句顺序不要反了 
			father[fa] = fb;  //小集合并入大集合
		} else {
			father[fa] += father[fb];
			father[fb] = fa;
		}
	}
}

int main() {
	int n, m, a, b;
	//while (~scanf("%d%d", &n, &m)) {
	while (~scanf("%d", &m)) {
		if (!m) {
			printf("1\n");
			continue;
		}
		n = -1;
		//初始化数组元素 
		fill(father, father + maxn, -1);
		for (int i = 0; i < m; ++i) {
			scanf("%d%d", &a, &b);
			Union(a, b);
			n = max(n, a);  //动态更新最大值 
			n = max(n, b);
		}
		int sum = -1;
		for (int i = 1; i <= n; ++i) {
			//注意这里要取个绝对值~ 
			if (father[i] < -1) sum = max(sum, abs(father[i]));
		}
		printf("%d\n", sum);
	}
	return 0;
}