北京大学机试 Jungle Roads 需要二刷 *最小生成树的升级版

基本思想：

感觉以后要注意一下，最小生成树里面有几个坑：

1.自环问题；

2.两节点间拥有多重路径；

关键点：

Prim基础版本无需考虑自环问题；

#include<iostream>
#include<stdlib.h>
#include<vector>
#include<string>
using namespace std;

const int maxn = 28;
const int INF = 100000000;
int ma[maxn][maxn];
bool vis[maxn];
int dis[maxn];

int num,n;

int prim() {
	dis[0] = 0;
	int ans = 0;
	for (int u = 0; u < n; u++) {
		int index = -1;
		int Min = INF;
		for (int i = 0; i < n; i++) {
			if (!vis[i] && dis[i] < Min) {
				index = i;
				Min = dis[i];
			}
		}
		if (index == -1)
			return -1;
		vis[index] = true;
		ans += dis[index];
		for (int i = 0; i < n; i++) {
			if (!vis[i] && ma[index][i] != INF && dis[i] > ma[index][i]) {
				dis[i] = ma[index][i];
			}
		}
	}
	return ans;
}

void init() {
	fill(ma[0], ma[0] + maxn * maxn, INF);
	fill(vis, vis + maxn, false);
	fill(dis, dis + maxn, INF);
}
int main() {
	while (cin >> num) {
		if (num == 0)
			return 0;
		init();
		char a, b;
		int m, d;
		char most = 'A';
		for (int i = 0; i < num-1; i++) {
			cin >> a >> m;
			if (a > most)
				most = a;
			for (int j = 0; j < m; j++) {
				cin >> b >> d;
				if (most < b)
					most = b;
				if(ma[a - 'A'][b - 'A']>d)
					ma[a - 'A'][b - 'A'] = ma[b - 'A'][a - 'A'] = d;
			}
		}
		n = most - 'A' + 1;
		int ans = prim();
		cout << ans << endl;
	}
	return 0;
}