学步园

传送门：【POJ】2114 Boatherds

题目分析：2333又水了一道题。。

问是否存在长度等于K的路径。就是将统计小于等于K的换成统计等于K的条数，只要最后统计出来的等于K的数量大于0就是存在。其他一点没变，还是那个点分治，还是曾经的味道~

代码如下：

#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std ;

typedef long long LL ;

#define travel( e , H , u ) for ( Edge* e = H[u] ; e ; e = e -> next )
#define rep( i , a , b ) for ( int i = ( a ) ; i <  ( b ) ; ++ i )
#define rev( i , a , b ) for ( int i = ( a ) ; i >= ( b ) ; -- i )
#define FOR( i , a , b ) for ( int i = ( a ) ; i <= ( b ) ; ++ i )
#define clr( a , x ) memset ( a , x , sizeof a )
#define cpy( a , x ) memcpy ( a , x , sizeof a )

const int MAXN = 10005 ;
const int MAXE = 20005 ;

struct Edge {
	int v , c ;
	Edge* next ;
} E[MAXE] , *H[MAXN] , *edge ;

bool vis[MAXN] ;
int dep[MAXN] ;
int siz[MAXN] ;
int num[MAXN] ;
int S[MAXN] ;
int weight ;
int root ;
int top ;
int ans ;
int n , K ;

void clear () {
	edge = E ;
	clr ( H , 0 ) ;
}

void addedge ( int u , int v , int c ) {
	edge -> v = v ;
	edge -> c = c ;
	edge -> next = H[u] ;
	H[u] = edge ++ ;
}

void get_root ( int u , int fa = 0 ) {
	siz[u] = 1 ;
	num[u] = 0 ;
	travel ( e , H , u ) {
		int v = e -> v ;
		if ( !vis[v] && v != fa ) {
			get_root ( v , u ) ;
			siz[u] += siz[v] ;
			num[u] = max ( num[u] , siz[v] ) ;
		}
	}
	num[u] = max ( num[u] , weight - siz[u] ) ;
	if ( num[u] < num[root] ) root = u ;
}

void get_dep ( int u , int fa = 0 ) {
	if ( dep[u] <= K ) S[top ++] = dep[u] ;
	siz[u] = 1 ;
	travel ( e , H , u ) {
		int v = e -> v ;
		if ( !vis[v] && v != fa ) {
			dep[v] = dep[u] + e -> c ;
			get_dep ( v , u ) ;
			siz[u] += siz[v] ;
		}
	}
}

int get_num ( int u , int initial_dep ) {
	top = 0 ;
	dep[u] = initial_dep ;
	get_dep ( u ) ;
	sort ( S , S + top ) ;
	int l = 0 , r = top - 1 , res = 0 ;
	while ( l < r ) {
		if ( S[l] + S[r] > K ) -- r ;
		else if ( S[l] + S[r] < K ) ++ l ;
		else {
			if ( S[l] == S[r] ) {
				res += ( r - l + 1 ) * ( r - l ) / 2 ;
				break ;
			}
			int i = l , j = r ;
			while ( S[l] == S[i] ) ++ i ;
			while ( S[r] == S[j] ) -- j ;
			res += ( i - l ) * ( r - j ) ;
			l = i , r = j ;
		}
	}
	return res ;
}

void dfs ( int u ) {
	vis[u] = 1 ;
	ans += get_num ( u , 0 ) ;
	travel ( e , H , u ) {
		int v = e -> v ;
		if ( !vis[v] ) {
			ans -= get_num ( v , e -> c ) ;
			root = 0 ;
			weight = siz[v] ;
			get_root ( v ) ;
			dfs ( root ) ;
		}
	}
}

void solve () {
	int x , c ;
	clear () ;
	FOR ( i , 1 , n ) {
		while ( ~scanf ( "%d" , &x ) && x ) {
			scanf ( "%d" , &c ) ;
			addedge ( i , x , c ) ;
			addedge ( x , i , c ) ;
		}
	}
	while ( ~scanf ( "%d" , &K ) && K ) {
		clr ( vis , 0 ) ;
		ans = root = 0 ;
		weight = num[0] = n ;
		get_root ( 1 ) ;
		dfs ( root ) ;
		printf ( ans ? "AYE\n" : "NAY\n" ) ;
	}
	printf ( ".\n" ) ;
}

int main () {
	while ( ~scanf ( "%d" , &n ) && n ) solve () ;
	return 0 ;
}