PTA 7-10 公路村村通 （30 分）

#include <bits/stdc++.h>

using namespace std;

const int inf = 999999;

int n, m; //N爲城鎮數目，M爲候選道路數目
int g[1000][1000]; //g[][]用來存儲城鎮之間的距離
int x, y, z; //初始化時x,y用作一條路徑兩端的城鎮，z用作路徑的長度 
int cost[1000]; //cost[][]用來作爲最短路徑的每段長度

int prime();
 
int main()
{
	#ifdef ONLINE_JUDGE
	#else
	freopen("in.txt","r",stdin);
	#endif
	
	cin >> n >> m;
	
	for(int i = 1; i <= n; i++){
		
		for(int j = 1; j <= n; j++){
			
			g[i][j] = g[j][i] = inf;
			
		} 
	}
	
	while(m--){
		
		cin >> x >> y >> z;
		g[x][y] = g[y][x] = z; 
		
	}

	for(int i = 1; i <= n; i++){
		
		cost[i] = g[1][i];
		 
	}	
	
	cout << prime() << endl;
	
	return 0;
}

int prime()
{
	int sum = 0;
	cost[1] = -1;
	
	int i;
	
	for(i = 1; i < n; i++){
		
		int minimum = 999999;
		int k = -1;
		
		for(int j = 1; j <= n; j++){
			
			if(cost[j] != -1 && cost[j] < minimum){
				
				minimum = cost[j];
				k = j;
				
			}
						
		}
		
		if(k != -1){
			
			sum += cost[k];
			cost[k] = -1;
			
			for(int j = 1; j <= n; j++){
				
				if(g[k][j] < cost[j]) cost[j] = g[k][j];
				
			}
			
		}
	
	}
	
	for( i = 1; i <= n; i++){
		
		if(cost[i] != -1) break;
		
	}
	
	if(i <= n) return -1;
	else return sum;

}