hdu2196

基本的树形dp，需要dfs三次，第一次求每个点最远的后代，第二次和第三次每个点的孩子分别从左到右和从右到左遍历。

#include <cstdio>
#include <vector>
using namespace std;

#define D(x) 

const int MAX_N = (int)(1e4) + 5;

int n;
vector <pair<int, int> > edge[MAX_N];
int child[MAX_N];
int father_left[MAX_N];
int father_right[MAX_N];

void dfs_child(int father, int u)
{
    child[u] = 0;
    for (int i = 0; i < (int)edge[u].size(); i++)
    {
        int v = edge[u][i].first;
        int w = edge[u][i].second;
        if (v != father)
        {
            dfs_child(u, v);
            child[u] = max(child[u], child[v] + w);
        }
    }
}

void dfs_father_left(int father, int u, int up)
{
    father_left[u] = up;
    int temp = 0;
    for (int i = 0; i < (int)edge[u].size(); i++)
    {
        int v = edge[u][i].first;
        int w = edge[u][i].second;
        if (v != father)
        {
            dfs_father_left(u, v, max(up, temp) + w);
            temp = max(temp, w + child[v]);
        }
    }
}

void dfs_father_right(int father, int u, int up)
{
    father_right[u] = up;
    int temp = 0;
    for (int i = (int)edge[u].size() - 1; i >= 0; i--)
    {
        int v = edge[u][i].first;
        int w = edge[u][i].second;
        if (v != father)
        {
            dfs_father_right(u, v, max(up, temp) + w);
            temp = max(temp, w + child[v]);
        }
    }
}

int main()
{
    while (scanf("%d", &n) != EOF)
    {
        for (int i = 1; i <= n; i++)
        {
            edge[i].clear();
        }
        for (int i = 2; i <= n; i++)
        {
            int a, b;
            scanf("%d%d", &a, &b);
            edge[i].push_back(make_pair(a, b));
            edge[a].push_back(make_pair(i, b));
        }
        dfs_child(-1, 1);
        dfs_father_left(-1, 1, 0);
        dfs_father_right(-1, 1, 0);
        for (int i = 1; i <= n; i++)
        {
            D(printf("%d %d %d
", child[i], father_left[i], father_right[i]));
            printf("%d
", max(child[i], max(father_left[i], father_right[i])));
        }
    }
    return 0;
}