UVa

POJ

Description

求苗条度最小的生成树

苗条度指该生成树的最大边 - 最小边

Algorithm

Kruskal变形

先sort

然后枚举最小边 构建最小生成树

Hint

UVa JAVA的RE 不知道为何= =

POJ的JAVA才1.5 λ表达式是JAVA 1.8才有的 然后本人才疏学浅 除了λ表达式以外不会写自定义sort比较 所以CE = =

C++就过了

会了C++ 的INF写法

#include <climits>

这样就有

INT_MAX这个常量了

Code

#include <cstdio>
#include <climits>
#include <algorithm>
using namespace std;
struct Edge
{
  int u, v, w;
  Edge(){}
  Edge(int u, int v, int w) : u(u), v(v), w(w) {}
};
const int MAX_M = 109 * 108 / 2;
const int MAX_N = 100 + 9;
int n;
bool comp(Edge a, Edge b)
{
  if (a.w < b.w) return true; else return false;
}
struct UnionFind
{
  int n;
  int par[MAX_N];
  int rank[MAX_N];
  UnionFind(int nn)
  {
    n = nn;
    for (int i = 0; i < n; i++) {
      par[i] = i;
      rank[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 (x == y) return;
    if (rank[x] < rank[y]) {
      par[x] = y;
    } else {
      par[y] = x;
      if (rank[x] == rank[y]) rank[x]++;
    }
  }
  bool same(int x, int y) {
    return find(x) == find(y);
  }
  void print()
  {
    for (int i = 0; i < n; i++) {
      printf("%d ", par[i]);
    }
  }
  int total()
  {
    int s = 0;
    for (int i = 0; i < n; i++) {
      if (par[i] == i) s++;
    }
    return s;
  }
};
void solve()
{
  int m;
  scanf("%d", &m);
  Edge edges[MAX_M];
  for (int i = 0; i < m; i++) {
    int u, v, w;
    scanf("%d%d%d", &u, &v, &w);
    edges[i] = Edge(u - 1, v - 1, w);
  }
  sort(edges, edges + m, comp);
  UnionFind unionFind(n);
  const int INF = INT_MAX / 2;
  int ans = INF;
  for (int i = 0; i < m; i++) {
    UnionFind unionFind = UnionFind(n);
    int e = 0;
    for (int j = i; j < m; j++) {
      if (!unionFind.same(edges[j].u, edges[j].v)) {
        unionFind.unite(edges[j].u, edges[j].v);
        e++;
        if (e == n - 1) {
          ans = min(edges[j].w - edges[i].w, ans);
          break;
        }
      }
    }
  }
  if (ans == INF) ans = -1;
  printf("%d\n", ans);
}
int main()
{
//  freopen("input.txt", "r", stdin);
  for (;;) {
    scanf("%d", &n);
    if (n == 0) break;
    solve();
  }
}