P1111-修复公路

  1 #include <bits/stdc++.h>
  2 #define _for(i,a,b) for(int i = (a);i < b;i ++)
  3 #define _rep(i,a,b) for(int i = (a);i > b;i --)
  4 #define INF 0x3f3f3f3f
  5 #define MOD 1000000007
  6 typedef long long ll;
  7 using namespace std;
  8 inline ll read()
  9 {
 10     ll ans = 0;
 11     char ch = getchar(), last = ‘ ‘;
 12     while(!isdigit(ch)) last = ch, ch = getchar();
 13     while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - ‘0‘, ch = getchar();
 14     if(last == ‘-‘) ans = -ans;
 15     return ans;
 16 }
 17 inline void write(ll x)
 18 {
 19     if(x < 0) x = -x, putchar(‘-‘);
 20     if(x >= 10) write(x / 10);
 21     putchar(x % 10 + ‘0‘);
 22 }
 23 
 24 const int maxn = 100039;
 25 int par[maxn];
 26 int high[maxn];
 27 int cnt;
 28 void init(int n)
 29 {
 30     _for(i,1,n+1)
 31     {
 32         par[i] = i;
 33         high[i] = 0;
 34     }
 35     cnt = n-1;
 36 }
 37 
 38 int find(int x)
 39 {
 40     return par[x] == x ? x : par[x] = find(par[x]);
 41 }
 42 
 43 void unite(int x,int y)
 44 {
 45     x = find(x);
 46     y = find(y);
 47     if(x==y) return ;
 48     cnt --;
 49     if(high[x]<high[y])
 50         par[x] = y;
 51     else
 52     {
 53         par[y] = x;
 54         if(high[x]==high[y])
 55             high[x] ++;
 56     }
 57 }
 58 
 59 bool same(int x,int y)
 60 {
 61     return find(x) == find(y);
 62 }
 63 int N,M;
 64 struct edge
 65 {
 66     int from;
 67     int to;
 68     int cost;
 69     bool operator < (edge b)
 70     {
 71         return this->cost<b.cost;
 72     }
 73 };
 74 edge x[maxn];
 75 bool C(int d)
 76 {
 77     init(N);
 78     _for(i,1,M+1)
 79         if(d>=x[i].cost)
 80             unite(x[i].from,x[i].to);
 81     return cnt == 0;
 82 }
 83 int solve()
 84 {
 85     sort(x+1,x+M+1);
 86     int lb = 0,ub = 110000;
 87     while(lb < ub)
 88     {
 89         int mid =  lb+(ub-lb)/2;
 90         if(C(mid)) ub = mid;
 91         else lb = mid+1;
 92     }
 93     return lb;
 94 }
 95 
 96 int main()
 97 {
 98     N = read(), M = read();
 99     int maxx;
100     _for(i,1,M+1)
101     {
102         x[i].from = read();
103         x[i].to = read();
104         x[i].cost = read();
105         maxx = max(maxx,x[i].cost);
106     }
107     int rnt = solve();
108     if(rnt>maxx)
109         printf("-1\n");
110     else
111         printf("%d\n",rnt);
112     return 0;
113 }