POJ 2395 - Out of Hay(最小生成树)

题目:

http://poj.org/problem?id=2395

思路:

最小生成数,WA了几遍之后去看discuss发现有重边, prim和kruskal两种算法都写了,当模板咯=.= kuskal 算法会快些.

CODE:

kruskal (63ms)

#include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;
const int INF = 1e9;
const int maxn = 2000*2000;
struct edge{
int u, v, cost;
}eg[maxn];
int V, E;
int par[2005];

bool cmp(edge a, edge b)
{
return a.cost < b.cost;
}
void init(int n)
{
for(int i = 0; i < n; ++i) par[i] = i;
}
int find(int x)
{
if(par[x] == x) return x;
else {
return par[x] = find(par[x]);
}
}
void unite(int x, int y)
{
x = find(x);
y = find(y);
if(y < x) par[x] = y;
else if(y > x) par[y] = x;
}
bool same(int x, int y)
{
return find(x) == find(y);
}
int kruskal()
{
sort(eg, eg + E, cmp);
init(V);
int ans = 0;
for(int i = 0; i < E; ++i) {
edge e = eg[i];
if(!same(e.u, e.v)) {
unite(e.u, e.v);
ans = max(ans, e.cost);
}
}
return ans;
}
int main()
{
//freopen("in", "r", stdin);
while(~scanf("%d %d", &V, &E)) {
for(int i = 0; i < E; ++i) {
scanf("%d %d %d", &eg[i].u, &eg[i].v, &eg[i].cost);
}
printf("%d\n", kruskal());
}
return 0;
}

prim (141ms)

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;
const int maxn = 2005;
const int INF = 1e9;
int cost[maxn][maxn], d[maxn], n;
bool used[maxn];

int prim()
{
fill(d, d + n + 1, INF);
memset(used, 0, sizeof(used));
d[1] = 0;
int ans = -1;
while(1) {
int v = -1;
for(int i = 1; i <= n; ++i) {
if(!used[i] && (v == -1 || d[i] < d[v])) v = i;
}
if(v == -1) break;
used[v] = 1;
ans = max(ans, d[v]);
for(int i = 1; i <= n; ++i) {
d[i] = min(d[i], cost[v][i]);
}
}
return ans;
}

int main()
{
//freopen("in", "r", stdin);
int m;
while(~scanf("%d %d", &n, &m)) {
for(int i = 0; i <= n; ++i) {
for(int j = 0; j <= n; ++j) cost[i][j] = INF;
}

int a, b, c;
while(m--) {
scanf("%d %d %d", &a, &b, &c);
cost[a][b] = min(c, cost[a][b]);
cost[b][a] = min(c, cost[b][a]);
}
int ans = prim();
printf("%d\n", ans);
}
return 0;
}