【ZOJ月赛】【树形DP】【I.Destroy】

【题目来源】http://acm.zju.edu.cn/onlinejudge/showContestProblem.do?problemId=4937

【个人体会】这个题目刚开始看的时候我以为是树的中心点+最小割。但是最小割那边我很怕会超时，因此一开始就没敢打，后来发现可以使用树形DP在O(N)的级别中解决这个问题，因此思路很明确，首先是求树的中心点，然后是树形DP，总级别也是O(N)，可能带点常数，但无关紧要。

【题目解析】思路分为两块，第一块是求树的中心点，有经典的O(N)级别的算法。第二块是树形DP，DP[U]表示的是以U为根的子树中，切断所有叶子节点与根节点的联系，所要付出的那个最大边的代价。因此，对于U和U的某个儿子V，在cost[U,V]和DP(V)中选一个小的，记为min{cost[U,V], DP(V)}，最后在所有的min{cost[U,V], DP(V)}中选一个max用来更新DP(U)

【代码如下】

#include <iostream>
#include <cstdio>
#include <climits>
#include <cstdlib>
#include <cstring>
#include <vector>
#include <deque>
#include <stack>
#include <queue>
#include <algorithm>

#define rep(i, x) for (int i = 1; i <= x; ++i)
#define rept(i, x) for (int i = 0; i < x; ++i)
#define pb push_back
#define ppf pop_front
#define mp make_pair
#define pf push_front
#define x first
#define y second

#define FILE_IO
#define DEBUG

using namespace std;

typedef long long int64;

const int Max = 10001, MAX = 1e16 + 1;

struct edge
{
    int v, c, next;
    int64 w;
}E[Max * 2];

bool hash[Max];
int N, Cnt, H[Max], Root;
int64 Dp[Max], up[Max], down[Max][3];

void Clear()
{
    Cnt = 0;
    memset(H, 0, sizeof(H));
    memset(up, 0, sizeof(up));
    memset(down, 0, sizeof(down));
}

int64 Treedp_Dp(int i)
{
    if (Dp[i] != -1) return Dp[i];
    int64 t1 = MAX, t2 = 0;
    bool flag = false;
    for (int j = H[i], v; j; j = E[j].next)
    {
        v = E[j].v;
        if (!hash[v]) hash[v] = flag = true;
        else continue;
        t1 = min(Treedp_Dp(v), E[j].w), t2 = max(t2, t1);
    }
    if (!flag) return MAX;
    Dp[i] = t2;
    return t2;
}

void Treedp()
{
    memset(Dp, -1, sizeof(Dp));
    memset(hash, 0, sizeof(hash)); hash[Root] = true;
    printf("%lld\n", Treedp_Dp(Root));
}

void Center_find()
{
    int64 t1 = 0, t2 = MAX;
    for (int i = 1; i <= N; ++i)
    {
        t1 = max(up[i], down[i][1]);
        if (t1 < t2) t2 = t1, Root = i;
    }
}

void Center_upFind(int u, int fa, int64 dis)
{
    if (u != fa)
    {
        up[u] = up[fa] + dis;
        int64 tmp = 0;
        if (down[fa][1] == down[u][1] + dis) tmp = down[fa][2] + dis;
        else tmp = down[fa][1] + dis;
        if (tmp > up[u]) up[u] = tmp;
    }
    for (int j = H[u], v; j; j = E[j].next)
    {
        v = E[j].v;
        if (hash[v]) continue;
        hash[v] = true;
        Center_upFind(v, u, E[j].c);
    }
}

void Center_downFind(int u)
{
    for (int j = H[u], v; j; j = E[j].next)
    {
        v = E[j].v;
        if (hash[v]) continue;
        hash[v] = true;
        Center_downFind(v);
        if (down[v][1] + E[j].c > down[u][1])
        {
            down[u][2] = down[u][1];
            down[u][1] = down[v][1] + E[j].c;
        }
        else if (down[v][1] + E[j].c > down[u][2]) down[u][2] = down[v][1] + E[j].c;
    }
}

void Center()
{
    memset(hash, 0, sizeof(hash)); hash[1] = true;
    Center_downFind(1);
    memset(hash, 0, sizeof(hash)); hash[1] = true;
    Center_upFind(1, 1, 0);
    Center_find();
}

inline void edgeAdd(int &x, int &y, int &c, int &w)
{
    E[++ Cnt].next = H[x], H[x] = Cnt, E[Cnt].v = y, E[Cnt].c = c, E[Cnt].w = w;
}

void Init()
{
    for (int i = 1, x, y, c, w; i <= N - 1; ++i)
    {
        scanf("%d%d%d%d", &x, &y, &c, &w);
        edgeAdd(x, y, c, w);
        edgeAdd(y, x, c, w);
    }
}

int main()
{
    #ifdef FILE_IO
    //freopen("test.in", "r", stdin);
    #endif // FILE_IO
    while (scanf("%d", &N) != EOF)
    {
        Init();
        Center();
        Treedp();
        Clear();
    }
    return 0;
}