@(XSY)[分塊, 倍增]
Description
There's a new trend among Bytelandian schools. The "Byteland Touristic Bureau" has developed a new project for the high-schoolers. The project is so-called "Children's Trips".
The project itself is very simple: there are some touristic routes in Byteland, and N touristic campuses (numbered from 1 to N). For the sake of economy, there are exactly N-1 road between them. Of course, even having this given, it is possible to travel from any touristic campus to any other one. Moreover, for the sake of safety, each road is no longer than 2 kilometers.
When some student wants to travel, he first chooses his starting campus - he is been delivered there (say) by a helicopter. He chooses his final destination campus as well. From his final destination, he will be transported to his home by (say) a helicopter, again. So that pupil won't travel any extra distance by foot. When the start and the finish are chosen and the pupil is delivered, he starts his moving by the only route. None of pupils is infinitely strong, so first the pupil looks at the map of the touristic routes, and then he chooses the furthest campus on his way that he can reach during the current day (by safety regulations, it is strictly prohibited to sleep not at the campus because there can be a little trouble with werewolves), and moves there. Then the new day begins, and it repeats till the moment when the destination is reached.
Of course, not all the students created equal. Somebody is good in math, somebody in English, somebody in PE. So it's quite natural that all high-schoolers has different strengths.
We call the strength is the maximal number of kilometers that the pupil can pass in a day. And now you're given a lot of queries from the children. For every query, you are given the starting campus, the final campus and the strength. You are requested to calculate the number of days for every trip. The map of the campuses and the distances between them will be given to you as well.
The first line of input contains the integer \(N\), denoting the number of campuses.
The next \(N-1\) lines contain triples of the form \(X\) \(Y\) \(D\) with the meaning that there is a road between the X-th and the Y-th campus with the length \(D\) kilometers.
Then there is a line with a single integer \(M\), denoting the number of queries.
Then, there are \(M\) lines with the triples of the form \(S\) \(F\) \(P\) with the meaning that the trip starts at the campus \(S\), ends at the campus \(F\) and the student has the strength of \(P\).
\[Output
For every query, please output the number of days on a separate line.
Constraints
$1 ≤ N, M ≤ 100000$
$1 ≤ X, Y, S, F ≤ N$
$1 ≤ D ≤ 2$
$2 ≤ P ≤ 2*N$
##Example
###Input:
```dos
5
1 2 1
2 3 2
1 4 2
4 5 1
5
1 5 3
1 3 2
2 5 4
1 2 10
4 5 2
```
###Output:
```dos
1
2
1
1
1
```
##Solution
翻譯一下題意:
>有一棵每条边的边权分别为$1$或$2$的共有$n$个节点的树, 对于一个询问, 给出起点$u$和终点$v$, 以及每一天最多走的路程$k$. 规定每天的结束点必须在树的节点上, 问最少要几天走完所有路程.
由於邊的權值只能為$1$或$2$, 因此可以考慮採用時間複雜度帶根號的算法.
>对$p$分情况进行讨论
>1. $p \ge \sqrt{n}$
>最多总共走$\sqrt{n}$天, 对于每一天倍增最远可以走到哪里即可, 并统计走的天数.
>2. $p \le sqrt(n)$
>对于每次询问的$p$, 暴力求出从每个点出发, 最多走$p$距离最远可以到达的最远位置. 再用倍增求出$f[i][j]$数组, 表示从点$i$出发, 走$2 ^ j$天可以到达的最远位置. 最后跑一次倍增解决.
>注意在进行这种计算之前, 先要将$p$从小到大排序. 这样每次计算就可以省去之前已经进行过的部分. 同时, 对于两个相同的$p$值, 不要重复求$f$数组, 否则会超时.
>时间复杂度$O \left( n * \sqrt{n} * log(n) \right)$
```cpp
#include<cstdio>
#include<cctype>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;
inline int read()
{
int x = 0, flag = 1;
char c;
while(! isdigit(c = getchar()))
if(c == '-')
flag *= - 1;
while(isdigit(c))
x = x * 10 + c - '0', c = getchar();
return x * flag;
}
void println(int x)
{
if(x < 0)
putchar('-'), x *= - 1;
if(x == 0)
putchar('0');
int ans[1 << 5], top = 0;
while(x)
ans[top ++] = x % 10, x /= 10;
for(; top; top --)
putchar(ans[top - 1] + '0');
putchar('\n');
}
const int N = 1 << 17, M = 1 << 17;
int n;
int head[N];
int top;
struct edge
{
int v, w, next;
}G[N << 1];
inline void addEdge(int u, int v, int w)
{
G[top].v = v, G[top].w = w, G[top].next = head[u];
head[u] = top ++;
}
struct data
{
int u, dis;
}st[N][17];
int dep[N];
int disToRoot[N];
void dfs(int u, int pre, int w)
{
st[u][0].u = pre;
st[u][0].dis = w;
dep[u] = dep[pre] + 1;
disToRoot[u] = disToRoot[pre] + w;
for(int i = head[u]; ~ i; i = G[i].next)
if(G[i].v != pre)
dfs(G[i].v, u, G[i].w);
}
inline void getSt()
{
for(int i = 1; i < 17; i ++)
for(int j = 1; j <= n; j ++)
st[j][i].u = st[st[j][i - 1].u][i - 1].u,
st[j][i].dis = st[j][i - 1].dis + st[st[j][i - 1].u][i - 1].dis;
}
struct query
{
int id, s, t, p;
inline friend int operator <(query a, query b)
{
return a.p < b.p;
}
}a[M];
int getLca(int u, int v)
{
if(dep[u] < dep[v])
swap(u, v);
for(int i = 17 - 1; ~ i; i --)
if(dep[u] - (1 << i) >= dep[v])
u = st[u][i].u;
if(u == v)
return u;
for(int i = 17 - 1; ~ i; i --)
if(st[u][i].u != st[v][i].u)
u = st[u][i].u, v = st[v][i].u;
return st[u][0].u;
}
inline int climb(int &u, int lca, int p)
{
int ret = 0;
while(1)
{
if(disToRoot[u] - disToRoot[lca] < p)
break;
int left = p;
for(int i = 17 - 1; ~ i; i --)
if(left >= st[u][i].dis)
left -= st[u][i].dis, u = st[u][i].u;
ret ++;
}
return ret;
}
int ans[M];
int f[N][17];
inline int jump(int &u, int lca)
{
int ret = 0;
for(int i = 17 - 1; ~ i; i --)
if(dep[f[u][i]] > dep[lca])
ret += 1 << i, u = f[u][i];
return ret;
}
int main()
{
#ifndef ONLINE_JUDGE
freopen("childrenTrip.in", "r", stdin);
freopen("childrenTrip.out", "w", stdout);
#endif
n = read();
memset(head, - 1, sizeof(head));
top = 0;
for(int i = 1; i < n; i ++)
{
int u = read(), v = read(), w = read();
addEdge(u, v, w), addEdge(v, u, w);
}
dep[1] = - 1;
disToRoot[1] = 0;
dfs(1, 1, 0);
getSt();
int m = read();
for(int i = 0; i < m; i ++)
a[i].id = i, a[i].s = read(), a[i].t = read(), a[i].p = read();
sort(a, a + m);
int p;
for(p = 0; p < m; p ++)
if(a[p].p > (int)sqrt(n))
break;
for(int i = p; i < m; i ++)
{
int lca = getLca(a[i].s, a[i].t);
ans[a[i].id] = climb(a[i].s, lca, a[i].p) + climb(a[i].t, lca, a[i].p)
+ (disToRoot[a[i].s] + disToRoot[a[i].t] - 2 * disToRoot[lca] + a[i].p - 1) / a[i].p;
}
for(int i = 1; i <= n; i ++)
f[i][0] = i;
int last = 0;
for(int i = 0; i < p; i ++)
{
if(a[i].p != last)
{
for(int j = 1; j <= n; j ++)
{
int rest = a[i].p - (disToRoot[j] - disToRoot[f[j][0]]);
int u = f[j][0];
for(; ;)
{
rest -= st[u][0].dis;
u = st[u][0].u;
if(rest >= 0)
f[j][0] = u;
if(rest <= 0 || ! dep[u])
break;
}
}
for(int j = 1; j < 17; j ++)
for(int k = 1; k <= n; k ++)
f[k][j] = f[f[k][j - 1]][j - 1];
last = a[i].p;
}
int lca = getLca(a[i].s, a[i].t);
ans[a[i].id] = jump(a[i].s, lca) + jump(a[i].t, lca)
+ (disToRoot[a[i].s] + disToRoot[a[i].t] - 2 * disToRoot[lca] + a[i].p - 1) / a[i].p;
}
for(int i = 0; i < m; i ++)
println(ans[i]);
}
```\]
