uva 1395 Slim Span[最小生成树]

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<iostream>
#include<queue>
#include<vector>
#include<functional>
using namespace std;
typedef long long ll;
const int maxn = 1e4 + 10;
const double eps = 1e-9;
const int inf = 1 << 30;

int p[maxn], r[maxn];
int n, m;
struct E {
int x, y, l;
bool operator < (const E &t) const {
return l < t.l;
}
} e[maxn];

int findx(int x) {
return p[x] == x ? x : p[x] = findx(p[x]);
}

void Merge(int x, int y)
{
int fx,fy;
fx = findx(x);
fy = findx(y);
p[fx] = fy;
}

int kt(int i) {
int j = i + n - 2;
int ans = 0;
for(int k = 1; k <= n; k++) p[k] = k;
for(int k = i; k <= j; k++) {
int x = e[k].x, y = e[k].y;
if(findx(x) != findx(y)) Merge(x, y);
}
for(int k = j; k < m; k++) {
if(k != j) {
int x = e[k].x, y = e[k].y;
if(findx(x) != findx(y)) Merge(x, y);
}
int f = findx(1);
int gg = 1;
for(int g = 1; g <= n; g++) {
if(findx(g) != f) {gg = 0; break;}
}
if(gg) return e[k].l - e[i].l;
}
return inf;
}

int main() {
while(~scanf("%d %d", &n, &m) && n) {
for(int i = 0; i < m; i++) {
scanf("%d %d %d", &e[i].x, &e[i].y, &e[i].l);
}
sort(e, e + m);
int ans = inf;
for(int i = 0; i < m; i++) {
ans = min(ans, kt(i));
}
if(ans == inf) ans = -1;
printf("%d\n", ans);
}
return 0;
}