最小生成树的常用算法模板
生成树:有原图中的所有n个点,但只有n-1条边,且任意两个点能连通,不能有环
最小生成树:边权值之和最小的生成树
1、所有最小生成树算法
最小生成树问题一般都使用无向图,基本不会遇到有向图的问题。
2、朴素版 Prim 算法
稠密图就用Prim算法。
#include<iostream>
#include<cstring>
#include<algorithm>
using namespace std;
const int N = 510, INF = 0x3f3f3f3f;
int g[N][N];
int n, m;
int dist[N];
bool st[N];
int prim()
{
memset(dist, 0x3f, sizeof dist);
int res = 0;
for (int i = 0; i < n; i++)
{
int t = -1;
for (int j = 1; j <= n; j++)
if (!st[j] && (t == -1 || dist[t] > dist[j]))
t = j;
if (i && dist[t] == INF) return INF;
if (i) res += dist[t]; // 注意更新 res 的位置,一定要在更新 dist 数组之前。若先更新dist,可能存在边权更小的自环,导致 dist[t] 被自环更新
for (int j = 1; j <= n; j++) dist[j] = min(dist[j], g[t][j]);
st[t] = true;
}
return res;
}
int main()
{
cin >> n >> m;
memset(g, 0x3f, sizeof g);
while (m--)
{
int a, b, c;
cin >> a >> b >> c;
g[b][a] = g[a][b] = min(g[a][b], c);
}
int t = prim();
if (t == INF) cout << "impossible" << endl;
else cout << t << endl;
return 0;
}
3、克鲁斯卡尔算法(Kruskal)
#include<iostream>
#include<cstring>
#include<algorithm>
using namespace std;
const int N = 200010;
int n, m;
int p[N];
struct Edge
{
int a, b, w;
bool operator<(const Edge& W) const
{
return w < W.w;
}
}edges[N];
int find(int x)
{
if(p[x] != x) p[x] = find(p[x]);
return p[x];
}
int main()
{
cin >> n >> m;
for (int i = 0; i < m; i++)
{
int a, b, c;
cin >> a >> b >> c;
edges[i] = {a, b, c};
}
sort(edges, edges + m);
for (int i = 1; i <= n; i++) p[i] = i;
int res = 0, cnt = 0;
for (int i = 0; i < m; i++)
{
int a = edges[i].a, b = edges[i].b, w = edges[i].w;
a = find(a), b = find(b);
if (a != b)
{
p[a] = b;
res += w;
cnt++;
}
}
if (cnt < n - 1) cout << "impossible" << endl;
else cout << res << endl;
return 0;
}