对拍自取。

#include<bits/stdc++.h>
using namespace std;

inline bool rd ( int &k ){
    char c = getchar ();
    bool neg = false;
    while ( c != EOF && c != '-' && ( c < '0' || c > '9' ) )
        c = getchar ();
    if ( c == EOF ) return false;
    if ( c == '-' ){
        neg = true;
        k = 0;
    }else
        k = c - '0';
    c = getchar ();
    while ( c >= '0' && c <= '9' ){
        k = ( k << 1 ) + ( k << 3 ) + c - '0'; c = getchar ();
    }
    if ( neg ) k = -k;
    return true;
}
inline bool lrd ( long long &k ){
    char c = getchar ();
    bool neg = false;
    while ( c != EOF && c != '-' && ( c < '0' || c > '9' ) )
        c = getchar ();
    if ( c == EOF ) return false;
    if ( c == '-' ){
        neg = true;
        k = 0;
    }else
        k = c - '0';
    c = getchar ();
    while ( c >= '0' && c <= '9' ){
        k = ( k << 1 ) + ( k << 3 ) + c - '0'; c = getchar ();
    }
    if ( neg ) k = -k;
    return true;
}
inline void freo (){
    freopen ( "data.in", "r", stdin );
    freopen ( "data.out", "w", stdout );
}

const int N = 500001;
const int M = 1000001;
const int INF = 0x3f3f3f3f;
int vtot, pre [N], dia_tot;
int dis_in [N], dis_out [N], max_dis;
int dir [M], hed [N], val [M], dia [N], etot, nxt [M];
bool vis [N];

inline void new_e ( int u, int v, int k ){
    int i = ++etot;
    dir [i] = v;
    val [i] = k;
    nxt [i] = hed [u];
    hed [u] = i;
}
inline void bfs ( int start, bool use_pre ){
    for ( int i = 1; i <= vtot; ++i ) dis_in [i] = INF;
    queue <int> q;
    dis_in [start] = 0;
    q.push (start);
    while ( q.size () ){
        int from = q.front (); q.pop ();
        for ( int i = hed [from]; i; i = nxt [i] ){
            int to = dir [i];
            if ( dis_in [to] != INF ) continue;
            dis_in [to] = dis_in [from] + val [i];
            if ( use_pre ) pre [to] = from;
            q.push (to);
        }
    }
}
inline void diameter (){
    bfs ( 1, false );
    int start = 1;
    for ( int i = 2; i <= vtot; ++i )
        if ( dis_in [i] > dis_in [start] )
            start = i;
    bfs ( start,true );
    int end = start;
    for ( int i = 1; i <= vtot; ++i )
        if ( dis_in [i] > dis_in [end] )
            end = i;
    int temp = end;
    while ( temp ){
        dia [++dia_tot] = temp;
        temp = pre [temp];
    }
}
inline void dfs ( int from ){
    for ( int i = hed [from]; i; i = nxt [i] ){
        int to = dir [i];
        if ( vis [to] ) continue;
        vis [to] = true;
        dfs ( to );
        dis_out [from] = max ( dis_out [from], dis_out [to] + val [i] );
    }
}
inline int farthest (){
    for ( int i = 0; i <= dia_tot; ++i )
        vis [ dia[i] ] = true;
    for ( int i = 0; i <= dia_tot; ++i )
        dfs ( dia[i] );
    int out = -INF;
    for ( int i = 1; i <= dia_tot; ++i )
        out = max ( out, dis_out [ dia[i] ] );
    return out;
}

int main (){
    rd ( vtot );
    rd ( max_dis );
    for ( int i = 1; i < vtot; ++i ){
        int u, v;
        int k;
        rd ( u ); rd ( v );
        rd ( k );
        new_e ( u, v, k );
        new_e ( v, u, k );
    }
    diameter ();
    int max_out = farthest ();
    int min_ecc = INF;
    for ( int first = 1, second = 1; second <= dia_tot; ++first ){
        int first_node = dia [first];
        int second_node = dia [second];
        while ( second <= dia_tot && abs ( dis_in [first_node] - dis_in [second_node] ) <= max_dis ){
            min_ecc = min ( min_ecc, max ( max_out, 
            max ( abs(dis_in[dia[1]]-dis_in[first_node]), abs (dis_in[dia[dia_tot]]-dis_in[second_node]) ) ) );
            second ++;
            second_node = dia [second];
        }
    }
    printf ( "%d", min_ecc ); return 0;
}

没有讲解，可能以后会有。
（小声ACAC，时空限制好严格）