noip模拟赛 旅行

分析：一个贪心的想法是每次找到根的点权和最大的点进行操作，关键是怎么维护.每次找最大值，修改后会对这条链上每个点的子树上的点造成影响，可以用线段树来维护.找最大值就是区间求最大值嘛，对子树进行操作利用dfs序维护一下就好了.记录一下最大值的位置，每次从这个位置向上跳并对它的子树进行修改直到当前点的点权为0.

#include <cstdio>

#include <cstring>

#include <iostream>

#include <algorithm>

using namespace std;

const int maxn = ;

typedef long long ll;

int n, k, a[maxn], fa[maxn],head[maxn], to[maxn], nextt[maxn],tot = , d[maxn], l[maxn], r[maxn], pos[maxn], dfs_clock;

int maxx[maxn << ], tag[maxn << ], p[maxn << ];

ll ans;

void add(int x, int y)

{

    to[tot] = y;

    nextt[tot] = head[x];

    head[x] = tot++;

}

void dfs(int u, int from, int dist)

{

    d[u] = dist;

    l[u] = ++dfs_clock;

    pos[dfs_clock] = u;

    for (int i = head[u]; i; i = nextt[i])

    {

        int v = to[i];

        if (v != from)

        {

            dfs(v, u, dist + a[v]);

            fa[v] = u;

        }

    }

    r[u] = dfs_clock;

}

void pushup(int o)

{

    if (maxx[o * ] > maxx[o *  + ])

    {

        maxx[o] = maxx[o * ];

        p[o] = p[o * ];

    }

    else

    {

        maxx[o] = maxx[o *  + ];

        p[o] = p[o *  + ];

    }

}

void build(int o, int l, int r)

{

    if (l == r)

    {

        maxx[o] = d[pos[l]];

        p[o] = pos[l];

        return;

    }

    int mid = (l + r) >> ;

    build(o * , l, mid);

    build(o *  + , mid + , r);

    pushup(o);

}

void pushdown(int o)

{

    if (tag[o] != )

    {

        tag[o * ] += tag[o];

        tag[o *  + ] += tag[o];

        maxx[o * ] += tag[o];

        maxx[o *  + ] += tag[o];

        tag[o] = ;

    }

}

void update(int o, int l, int r, int x, int y,int v)

{

    if (x <= l && r <= y)

    {

        maxx[o] += v;

        tag[o] += v;

        return;

    }

    pushdown(o);

    int mid = (l + r) >> ;

    if (x <= mid)

        update(o * , l, mid, x, y, v);

    if (y > mid)

        update(o *  + , mid + , r, x, y, v);

    pushup(o);

}

void change(int x)

{

    while (a[x])

    {

        update(, , n, l[x], r[x], -a[x]);

        a[x] = ;

        x = fa[x];

    }

}

int main()

{

    scanf("%d%d", &n, &k);

    for (int i = ; i <= n; i++)

        scanf("%d", &a[i]);

    for (int i = ; i < n; i++)

    {

        int u, v;

        scanf("%d%d", &u, &v);

        add(u, v);

        add(v, u);

    }

    dfs(, , a[]);

    build(, , n);

    for (int i = ; i <= k; i++)

    {

        ans += maxx[];

        change(p[]);

    }

    printf("%lld\n", ans);

    return ;

}