[Usaco2013 Open]Yin and Yang

时间限制：10s 空间限制：128MB

题目描述

Farns (1 <= N <= 100,000) which are connected by N-1 edges such that he can reach any barn from any

other. Farmer John wants to choose a path which starts and ends at two different barns, such that he

does not traverse any edge twice. He worries that his path might be a little long, so he also wantst

o choose another "rest stop" barn located on this path (which is distinct from the start or the end)

. Along each edge is a herd of cows, either of the Charcolais (white hair) or the Angus (black hair)

variety. Being the wise man that he is, Farmer John wants to balance the forces of yin and yang tha

t weigh upon his walk. To do so, he wishes to choose a path such that he will pass by an equal numbe

r of Charcolais herds and Angus herds-- both on the way from the start to his rest stop, and on thew

ay from the rest stop to the end. Farmer John is curious how many different paths he can choose that

are "balanced" as described above. Two paths are different only if they consist of different setsof

edges; a path should be counted only once even if there are multiple valid "rest stop" locationsalo

ng the path that make it balanced. Please help determine the number of paths Farmer John can cho

输入格式

* Line 1: The integer N.

* Lines 2..N: Three integers a_i, b_i and t_i, representing the two barns that edge i connects. t_i

is 0 if the herd along that edge is Charcolais, and 1 if the herd is Angus.

输出格式

Line 1: One integer, representing the number of possible paths Farmer John can choose from.

样例输入

7
1 2 0
3 1 1
2 4 0
5 2 0
6 3 1
5 7 1
INPUT DETAILS: 
There are 7 barns and 6 edges. The edges from 1 to 2, 2 to 4 and 2 to 5 have Charcolais herds along 
them.

样例输出

1
OUTPUT DETAILS: 
No path of length 2 can have a suitable rest stop on it, so we can only consider paths of length 4. 
The only path that has a suitable rest stop is 3-1-2-5-7, with a rest stop at 2.

提示

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题目来源

Gold