hdu 2196 叶子节点最长距离（树DP）

http://www.cnblogs.com/kuangbin/archive/2012/08/28/2659915.html

求每个节点到叶子节点的最长距离

需要保存每个节点到叶子节点距离的最大值和次大值。第一次dfs求出到下面叶子节点的最大值，第二次dfs更新从父节点过来的最大值。

#include <iostream>
#include <string>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <stack>
#include <queue>
#include <cctype>
#include <vector>
#include <iterator>
#include <set>
#include <map>
#include <sstream>
using namespace std;

#define mem(a,b) memset(a,b,sizeof(a))
#define pf printf
#define sf scanf
#define spf sprintf
#define pb push_back
#define debug printf("!
")
#define MAXN 20000+5
#define MAX(a,b) a>b?a:b
#define blank pf("
")
#define LL long long
#define ALL(x) x.begin(),x.end()
#define INS(x) inserter(x,x.begin())
#define pqueue priority_queue
#define INF 0x3f3f3f3f

#define ls (rt<<1)
#define rs (rt<<1|1)

int n,m;

int ptr = 1,head[MAXN],a[MAXN],dp[MAXN][2],vis[MAXN];

int mx[MAXN],mx2[MAXN],vx[MAXN],vx2[MAXN];

struct node
{
    int y,val,next;
}tree[MAXN];

void add(int fa,int son,int val)
{
    tree[ptr].y = son;
    tree[ptr].val = val;
    tree[ptr].next = head[fa];
    head[fa] = ptr++;
}

void cg(int &x,int &y)
{
    int tmp = x;
    x = y;
    y = tmp;
}

void dfs(int root,int fa)
{
    for(int i=head[root];i!=-1;i=tree[i].next)
    {
        int y = tree[i].y;
        if(y == fa) continue;
        dfs(y,root);
        if(mx2[root] < mx[y]+tree[i].val)
        {
            mx2[root] = mx[y]+tree[i].val;
            vx2[root] = y;
            if(mx2[root]>mx[root])
            {
                swap(mx[root],mx2[root]);
                swap(vx[root],vx2[root]);
            }
        }
    }
}

void dfs2(int root,int fa)
{
    for(int i=head[root];i!=-1;i=tree[i].next)
    {
        int y = tree[i].y;
        if(y == fa) continue;
        if(y == vx[root])
        {
            if(tree[i].val + mx2[root] > mx2[y])
            {
                mx2[y] = tree[i].val + mx2[root];
                vx2[y] = root;
                if(mx2[y]>mx[y])
                {
                    swap(mx[y],mx2[y]);
                    swap(vx[y],vx2[y]);
                }
            }
        }
        else
        {
            if(tree[i].val + mx[root] > mx2[y])
            {
                mx2[y] = tree[i].val + mx[root];
                vx2[y] = root;
                if(mx2[y]>mx[y])
                {
                    swap(mx[y],mx2[y]);
                    swap(vx[y],vx2[y]);
                }
            }
        }
        dfs2(y,root);
    }
}

int main()
{
    int i,j,t,kase=1;
    while(~sf("%d",&n))
    {
        mem(tree,0);
        mem(head,-1);
        mem(mx,0);
        mem(mx2,0);
        mem(vx,0);
        mem(vx2,0);
        ptr = 1;
        int x,y;
        for(i=2;i<=n;i++)
        {
            sf("%d%d",&x,&y);
            add(x,i,y);
            add(i,x,y);
        }
        dfs(1,-1);
        dfs2(1,-1);
        for(i=1;i<=n;i++) pf("%d
",mx[i]);
    }
    return 0;
}