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 
      
       #include 
       
        using namespace std;#define D(x) const int MAX_N = (int)(1e4) + 5;int n;vector 
        
         
           > 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\n", child[i], father_left[i], father_right[i]));            printf("%d\n", max(child[i], max(father_left[i], father_right[i])));        }    }    return 0;}