Sonya and Ice Cream CodeForces - 1004E 树的直径, 贪心

题目链接

set维护最小值贪心, 刚开始用树的直径+单调队列没调出来...

#include <iostream>
#include <set>
#define REP(i,a,n) for(int i=a;i<=n;++i)
#define x first
#define y second
using namespace std;
typedef pair<int,int> pii;

const int N = 4e5+10, INF = 0x3f3f3f3f;
int n, k;
set<pii> s, g[N];

int main() {
    scanf("%d%d", &n, &k);
    REP(i,2,n) {
        int u, v, w;
        scanf("%d%d%d", &u, &v, &w);
        g[u].insert({v,w});
        g[v].insert({u,w});
    }
    REP(i,1,n) if (g[i].size()==1) {
        s.insert({g[i].begin()->y,i});
    }
    int ans = 0;
    while (n>k||s.size()>2) {
        ans = s.begin()->x;
        int x = s.begin()->y;
        s.erase(s.begin());
        int y = g[x].begin()->x;
        g[y].erase(g[y].lower_bound({x,0}));
        --n;
        if (g[y].size()==1) {
            s.insert({g[y].begin()->y+ans,y});
        }
    }
    printf("%d\n", ans);
}