例题11-2 苗条的生成树（Slim Span, ACM/ICPC Japan 2007, UVa1395)

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原题链接：https://vjudge.net/problem/UVA-1395
分类：图论
备注：kruskal算法,暴力

代码如下：

#include<cstdio>
#include<algorithm>
using namespace std;
const int inf = 10000 + 5;
const int maxn = 100 + 5;
const int maxm = 5000 + 5;
int u[maxm], v[maxm], w[maxm], r[maxm], fa[maxn], n, m, cnt[maxn];
bool cmp(const int i, const int j) {return w[i] < w[j];
}
int find(int x) {return fa[x] == x ? x : fa[x] = find(fa[x]);
}
int kruskal() {int ans = inf;for (int i = 1; i <= m; i++)r[i] = i;sort(r + 1, r + 1 + m, cmp);for (int i = 1; i <= n; i++) {fa[i] = i; cnt[i] = 1;}for (int i = 1; i <= m; i++) {int e = r[i];int x = find(u[e]), y = find(v[e]);if (x != y) {fa[y] = x;ans += w[e];cnt[x] += cnt[y];}}if (cnt[find(1)] != n)return -1;int maxL = m - n + 2;for (int L = 1; L <= maxL; L++) {for (int R = L + n - 2; R <= m; R++) {for (int i = 1; i <= n; i++) {fa[i] = i; cnt[i] = 1;}for (int i = L; i <= R; i++) {int e = r[i];int x = find(u[e]), y = find(v[e]);if (x != y) {fa[y] = x; cnt[x] += cnt[y];}}if (cnt[find(1)] == n) {ans = min(ans, w[r[R]] - w[r[L]]);break;//已经找到对应L可能的最小值}}}return ans;
}
int main(void) {while (~scanf("%d %d", &n, &m) && n) {for (int i = 1; i <= m; i++) scanf("%d %d %d", &u[i], &v[i], &w[i]);printf("%d\n", kruskal());}return 0;
}

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