HDU_5692_dfs序+线段树

http://acm.hdu.edu.cn/showproblem.php?pid=5692

这道题真的是看了题解还搞了一天，把每条路径后序遍历按1-n重新标号，储存每个点在哪些路径中出现过（l和r数组），然后转化成线段树来更新和取最大值。

注意，如果使用递归建线段树，数组要开4n才能保证不超。

刚开始更新的函数每一遍更新了所有子树，然后超时了，后来在tree中加了个add，保存子树需要增加的量，如果用到这个子树，在把这个量加起来，简直巧妙。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<vector>
#define INF 0x3f3f3f3f
#pragma comment(linker, "/STACK:1024000000,1024000000")
using namespace std;

vector<int> line[];
int n,m,cnt,l[],r[];
long long w[],init[];
struct segtree
{
    int left,right;
    long long maxx,add;
}tree[];

void dfs(int now,int pre,long long sum)
{
    sum += w[now];
    int flag = ;
    l[now] = INF;
    for(int i = ;i < line[now].size();i++)
    {
        int next = line[now][i];
        if(next == pre) continue;
        flag = ;
        dfs(next,now,sum);
        l[now] = min(l[now],l[next]);
    }
    init[cnt] = sum;
    r[now] = cnt++;
    if(flag)    l[now] = r[now];
}

void build(int pos,int l,int r)
{
    tree[pos].left = l;
    tree[pos].right = r;
    tree[pos].add = ;
    if(l == r)  tree[pos].maxx = init[l];
    else
    {
        int mid = (l+r)/;
        build(pos*,l,mid);
        build(pos*+,mid+,r);
        tree[pos].maxx = max(tree[pos*].maxx,tree[pos*+].maxx);
    }
}

void update(int pos,int l,int r,long long v)
{
    if(tree[pos].add != )
    {
        if(tree[pos].left != tree[pos].right)
        {
            tree[pos*].maxx += tree[pos].add;
            tree[pos*].add += tree[pos].add;
            tree[pos*+].maxx += tree[pos].add;
            tree[pos*+].add += tree[pos].add;
            tree[pos].add = ;
        }
    }
    if(tree[pos].left ==
       l && r == tree[pos].right)
    {
        tree[pos].maxx += v;
        tree[pos].add += v;
        return;
    }
    int mid = (tree[pos].left+tree[pos].right)/;
    if(r <= mid)    update(pos*,l,r,v);
    else if(l > mid)    update(pos*+,l,r,v);
    else
    {
        update(pos*,l,mid,v);
        update(pos*+,mid+,r,v);
    }
    tree[pos].maxx = max(tree[pos*].maxx,tree[pos*+].maxx);
}

long long getmax(int pos,int l,int r)
{
    if(tree[pos].add != )
    {
        if(tree[pos].left != tree[pos].right)
        {
            tree[pos*].maxx += tree[pos].add;
            tree[pos*].add += tree[pos].add;
            tree[pos*+].maxx += tree[pos].add;
            tree[pos*+].add += tree[pos].add;
            tree[pos].add = ;
        }
    }
    if(tree[pos].left == l && r == tree[pos].right) return tree[pos].maxx;
    int mid = (tree[pos].left+tree[pos].right)/;
    if(r <= mid)    return getmax(pos*,l,r);
    if(l > mid)     return getmax(pos*+,l,r);
    return max(getmax(pos*,l,mid),getmax(pos*+,mid+,r));
}

int main()
{
    int T;
    scanf("%d",&T);
    for(int z = ;z <= T;z++)
    {
        printf("Case #%d:\n",z);
        scanf("%d%d",&n,&m);
        for(int i = ;i < n;i++)    line[i].clear();
        for(int i = ;i < n-;i++)
        {
            int x,y;
            scanf("%d%d",&x,&y);
            line[x].push_back(y);
            line[y].push_back(x);
        }
        for(int i = ;i < n;i++)    scanf("%lld",&w[i]);
        cnt = ;
        dfs(,,);
        build(,,n);
        while(m--)
        {
            int op;
            scanf("%d",&op);
            if(op == )
            {
                int x,y;
                scanf("%d%d",&x,&y);
                long long temp = y-w[x];
                w[x] = y;
                update(,l[x],r[x],temp);
            }
            else
            {
                int x;
                scanf("%d",&x);
                printf("%lld\n",getmax(,l[x],r[x]));
            }
        }
    }
    return ;
}