給定各個村子之間的距離， 求最小生成樹

題解：

按題目給的輸入資料來說Prim可能更合適。但我更熟悉Kruskal, 所以強行用Kruskal。

程式碼：

#include <iostream>
#include <algorithm>
#include <cstring>
#include <cstdio>
#define MAXN 200

using namespace std;

struct Edge {int from, to, cost;}e[MAXN*MAXN];
int N, M;
int a[MAXN][MAXN];
int
 
 f[MAXN];
int ans;

void init() {
    for (int i = 1; i <= N; i++) f[i] = i;
}

int find(int x) {
    if (f[x] == x) return x;
    else return f[x] = find(f[x]);
}

bool same(int x, int y) {
    return find(x) == find(y);
}
void unite(int x, int y) {
    int fx = find(x);
    int fy = find(y);
    if
 
 (fx != fy) {
        f[fx] = fy;
    }
}

bool cmp(const Edge &a, const Edge &b) {
    return a.cost < b.cost;
}

int kruskal() {
    ans = 0;
    int k = 0;
    for (int i = 0; i < M && k <= N; i++) {
        if (!same(e[i].from, e[i].to)) {
            ans += e[i].cost;
            unite(e[i].from, e[i].to);
            k++;
        }
    }
    return
 
 ans;
}

int main(){
    while (scanf("%d", &N) == 1) {
        memset(a, 0, sizeof(a));
        M = 0;
        for (int i = 1; i <= N; i++)
            for (int j = 1; j <= N; j++) {
                scanf("%d", &a[i][j]);
                if (j > i && a[i][j]) {
                    e[M].from = i;
                    e[M].to = j;
                    e[M].cost = a[i][j];
                    M++;
                }
            }
        init();
        sort(e, e+M, cmp);
        ans = kruskal();
        printf("%d\n", ans);
    }
    return 0;
}