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Solution -「JOISC 2021」「LOJ #3495」聚會 2

\(\mathcal{Description}\)

  Link.

  給定一棵含 \(n\) 個結點的樹。稱點集 \(S\) 到結點 \(u\) 的會合距離為 \(\sum_{v\in S}\operatorname{dist}(u,v)\)。對於 \(|S|=1,2,\dots,n\),求使得滿足 \(S\) 一定且 \(S\)\(u\) 的會合距離最小時,可能選取的 \(u\) 的個數的最大值。

  \(n\le2\times10^5\)

\(\mathcal{Solution}\)

  可以發現,\(u\)\(S\) 在樹上的帶權重心。不難得到以下結論:

  • \(2\not\mid|S|\)
    ,答案為 \(1\)
  • 若存在多個合法的 \(u\),這些 \(u\) 構成一條樹鏈,且樹鏈上任意一條邊滿足:斷開這條邊,兩個聯通塊中屬於 \(S\) 的結點個數相等(這也印證了上個結論)。

  考慮“列舉”樹鏈更新答案。設一條樹鏈兩端結點構成的聯通塊大小為 \(s_x,s_y\),則滿足 \(|S|\le2\min\{s_x,s_y\}\)\(|S|\) 的答案都能用這條鏈的長度更新。注意到需要維護全域性聯通塊大小這一資訊,嘗試欽定全域性根後在有向樹上做點分治。每層分治更新答案時,可以欽定當前得到的聯通塊的小為某條樹鏈較小的一端,正反分別做一遍,用樹狀陣列就能維護。

  複雜度 \(\mathcal O(n\log^2n)\)

\(\mathcal{Code}\)

/* Clearink */

#include <cstdio>
#include <vector>
#include <algorithm>

#define rep( i, l, r ) for ( int i = l, rep##i = r; i <= rep##i; ++i )
#define per( i, r, l ) for ( int i = r, per##i = l; i >= per##i; --i )

inline int rint() {
    int x = 0, f = 1, s = getchar();
    for ( ; s < '0' || '9' < s; s = getchar() ) f = s == '-' ? -f : f;
    for ( ; '0' <= s && s <= '9'; s = getchar() ) x = x * 10 + ( s ^ '0' );
    return x * f;
}

template<typename Tp>
inline void wint( Tp x ) {
    if ( x < 0 ) putchar( '-' ), x = -x;
    if ( 9 < x ) wint( x / 10 );
    putchar( x % 10 ^ '0' );
}

inline void chkmax( int& a, const int b ) { a < b && ( a = b ); }

const int MAXN = 2e5, IINF = 0x3f3f3f3f;
int n, ecnt, head[MAXN + 5], ans[MAXN + 5];
int siz[MAXN + 5], par[MAXN + 5]; // for global root.
struct Edge { int to, nxt; } graph[MAXN * 2 + 5];
bool vis[MAXN + 5];

inline void link( const int u, const int v ) {
    graph[++ecnt] = { v, head[u] }, head[u] = ecnt;
    graph[++ecnt] = { u, head[v] }, head[v] = ecnt;
}

inline void findG( const int u, const int fa, int all, int& rt ) {
    static int tsiz[MAXN + 5], wgt[MAXN + 5];
    if ( !all ) all = tsiz[u];
    tsiz[u] = 1, wgt[u] = 0;
    for ( int i = head[u], v; i; i = graph[i].nxt ) {
        if ( !vis[v = graph[i].to] && v != fa ) {
            findG( v, u, all, rt ), tsiz[u] += tsiz[v];
            chkmax( wgt[u], tsiz[v] );
        }
    }
    chkmax( wgt[u], all - tsiz[u] );
    if ( !rt || wgt[u] < wgt[rt] ) rt = u;
}

inline void init( const int u ) {
    siz[u] = 1;
    for ( int i = head[u], v; i; i = graph[i].nxt ) {
        if ( ( v = graph[i].to ) != par[u] ) {
            par[v] = u, init( v );
            siz[u] += siz[v];
        }
    }
}

struct BinaryIndexTree {
    int val[MAXN + 5];
    inline void upd( int x, const int v ) {
        for ( x = n - x + 1; x <= n; x += x & -x ) chkmax( val[x], v );
    }
    inline int ask( int x ) {
        int ret = -IINF;
        for ( x = n - x + 1; x; x -= x & -x ) chkmax( ret, val[x] );
        return ret;
    }
    inline void clear( int x ) {
        for ( x = n - x + 1; x <= n; x += x & -x ) val[x] = -IINF;
    }
} bit;

inline void update( const int u, const int fa, const int d, const int sr ) {
    int su = u == par[fa] ? n - siz[u] : siz[u];
    chkmax( ans[su << 1], bit.ask( su ) + d );
    chkmax( ans[( su < sr ? su : sr ) << 1], d );
    for ( int i = head[u], v; i; i = graph[i].nxt ) {
        if ( !vis[v = graph[i].to] && v != fa ) {
            update( v, u, d + 1, sr );
        }
    }
}

inline void collect( const int u, const int fa, const int d ) {
    int su = u == par[fa] ? n - siz[u] : siz[u];
    bit.upd( su, d );
    for ( int i = head[u], v; i; i = graph[i].nxt ) {
        if ( !vis[v = graph[i].to] && v != fa ) {
            collect( v, u, d + 1 );
        }
    }
}

inline void clear( const int u, const int fa ) {
    int su = u == par[fa] ? n - siz[u] : siz[u];
    bit.clear( su );
    for ( int i = head[u], v; i; i = graph[i].nxt ) {
        if ( !vis[v = graph[i].to] && v != fa ) {
            clear( v, u );
        }
    }
}

inline void solve( const int u ) {
    vis[u] = true;
    
    static std::vector<int> son;
    son.clear();
    for ( int i = head[u], v; i; i = graph[i].nxt ) {
        if ( !vis[v = graph[i].to] ) {
            son.push_back( v );
        }
    }

    for ( int v: son ) {
        update( v, u, 1, par[v] == u ? siz[u] - siz[v] : siz[u] );
        collect( v, u, 1 );
    }
    for ( int v: son ) clear( v, u );
    std::reverse( son.begin(), son.end() );
    for ( int v: son ) {
        update( v, u, 1, par[v] == u ? siz[u] - siz[v] : siz[u] );
        collect( v, u, 1 );
    }
    for ( int v: son ) clear( v, u );

    for ( int i = head[u], v, rt; i; i = graph[i].nxt ) {
        if ( !vis[v = graph[i].to] ) {
            findG( v, u, 0, rt = 0 );
            solve( rt );
        }
    }
}

int main() {
    n = rint();
    rep ( i, 2, n ) link( rint(), rint() );
    
    int rt = 0; findG( 1, 0, n, rt );
    init( rt ); // let rt be global root.
    rep ( i, 1, n ) bit.val[i] = -IINF;

    solve( rt );
    
    per ( i, n - 1, 1 ) chkmax( ans[i], ans[i + 1] );
    rep ( i, 1, n ) wint( i & 1 ? 1 : ans[i] + 1 ), putchar( '\n' );
    return 0;
}