[IOI2011]Race【点分治】

题目链接

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  真的TLE了N久了，然后一直想不明白为什么用递归去作为clear清除操作，会TLE？（关键是一个测试点都没有过，让我自己算了N久的时间复杂度，以为自己写错了），debug了好久啊，然后想，是不是我的递归层数太多了，然后就这样被卡掉了，于是乎，我想，直接把答案存进去（相当于桶排了），然后我们直接对答案清除即可，这样就能把递归层数给降下来，就没有那么多次了。

  果然，这么一调之后就不再TLE了，直接WA了，得分55分，哦对了！没有输出-1的，改一下，75分。再想，果真是divide分治的时候给忘记对maxx = INF了，改完，A了，心情复杂…… 

//#pragma comment(linker, "/STACK:102400000,102400000")
#include <iostream>
#include <cstdio>
#include <cmath>
#include <string>
#include <cstring>
#include <algorithm>
#include <limits>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#define lowbit(x) ( x&(-x) )
#define pi 3.141592653589793
#define e 2.718281828459045
#define INF 0x3f3f3f3f
#define HalF (l + r)>>1
#define lsn rt<<1
#define rsn rt<<1|1
#define Lson lsn, l, mid
#define Rson rsn, mid+1, r
#define QL Lson, ql, qr
#define QR Rson, ql, qr
#define myself rt, l, r
#define MIN3(a, b, c) min(min(a, b), c)
using namespace std;
typedef unsigned long long ull;
typedef unsigned int uit;
typedef long long ll;
const int maxN = 2e5 + 7;
int N, K, head[maxN], cnt, all, ans, DT[1000007];
struct Eddge
{
    int nex, to, val;
    Eddge(int a=-1, int b=0, int c=0):nex(a), to(b), val(c) {}
}edge[maxN<<1];
inline void addEddge(int u, int v, int w)
{
    edge[cnt] = Eddge(head[u], v, w);
    head[u] = cnt++;
}
inline void _add(int u, int v, int w) { addEddge(u, v, w); addEddge(v, u, w); }
int siz[maxN], son[maxN], maxx, rt;
bool vis[maxN];
void findroot(int u, int fa)
{
    siz[u] = 1; son[u] = 0;
    for(int i=head[u], v; ~i; i=edge[i].nex)
    {
        v = edge[i].to;
        if(vis[v] || v == fa) continue;
        findroot(v, u);
        siz[u] += siz[v];
        son[u] = max(son[u], siz[v]);
    }
    son[u] = max(son[u], all - siz[u]);
    if(maxx > son[u]) { maxx = son[u]; rt = u; }
}
int Stop;
struct node
{
    int dis, deep;
    node(int a=0, int b=0):dis(a), deep(b) {}
}Stap[maxN];
void get_dis(int u, int fa, int dis, int deep)
{
    if(dis > K) return;
    Stap[++Stop] = node(dis, deep);
    for(int i=head[u], v; ~i; i=edge[i].nex)
    {
        v = edge[i].to;
        if(vis[v] || v == fa) continue;
        get_dis(v, u, dis + edge[i].val, deep + 1);
    }
}
void divide(int u)
{
    vis[u] = true;
    Stop = 0;
    int totsiz = all;
    for(int i=head[u], v, now_Stop; ~i; i=edge[i].nex)
    {
        v = edge[i].to;
        if(vis[v]) continue;
        now_Stop = Stop;
        get_dis(v, u, edge[i].val, 1);
        for(int j=now_Stop + 1; j <= Stop; j++) ans = min(ans, DT[K - Stap[j].dis] + Stap[j].deep);
        for(int j=now_Stop + 1; j <= Stop; j++) DT[Stap[j].dis] = min(DT[Stap[j].dis], Stap[j].deep);
    }
    for(int i=1; i<=Stop; i++) DT[Stap[i].dis] = INF;
    for(int i=head[u], v; ~i; i=edge[i].nex)
    {
        v = edge[i].to;
        if(vis[v]) continue;
        all = siz[v] > siz[u] ? totsiz - siz[u] : siz[v];
        maxx = INF;
        findroot(v, u);
        divide(rt);
    }
}
inline void init()
{
    cnt = 0; ans = INF; all = N;
    for(int i=1; i<=N; i++) head[i] = -1;
    for(int i=1; i<=N; i++) vis[i] = false;
    for(int i=1; i<=K; i++) DT[i] = INF;
    DT[0] = 0;
}
int main()
{
//    freopen("in.in", "r", stdin);
    scanf("%d%d", &N, &K);
    init();
    for(int i=1, u, v, w; i<N; i++)
    {
        scanf("%d%d%d", &u, &v, &w); u++; v++;
        _add(u, v, w);
    }
    maxx = INF; rt = 0;
    findroot(1, 0);
    divide(rt);
    printf("%d\n", ans == INF ? -1 : ans);
    return 0;
}