Xor-tree (Codeforces 430C)
Tags: Others, trees

Iahub is very proud of his recent discovery, propagating trees. Right now, he invented a new tree, called xor-tree. After this new revolutionary discovery, he invented a game for kids which uses xor-trees.The game is played on a tree having n nodes, numbered from 1 to n. Each node i has an initial value initi, which is either 0 or 1. The root of the tree is node 1.One can perform several (possibly, zero) operations on the tree during the game. The only available type of operation is to pick a node x. Right after someone has picked node x, the value of node x flips, the values of sons of x remain the same, the values of sons of sons of x flips, the values of sons of sons of sons of x remain the same and so on.The goal of the game is to get each node i to have value goali, which can also be only 0 or 1. You need to reach the goal of the game by using minimum number of operations.

Input: The first line contains an integer n (1 ≤ n ≤ 105). Each of the next n - 1 lines contains two integers ui and vi (1 ≤ ui, vi ≤ n; ui ≠ vi) meaning there is an edge between nodes ui and vi. The next line contains n integer numbers, the i-th of them corresponds to initi (initi is either 0 or 1). The following line also contains n integer numbers, the i-th number corresponds to goali (goali is either 0 or 1).

Output: In the first line output an integer number cnt, representing the minimal number of operations you perform. Each of the next cnt lines should contain an integer xi, representing that you pick a node xi.


Sample Inputs:
10
2 1
3 1
4 2
5 1
6 2
7 5
8 6
9 8
10 5
1 0 1 1 0 1 0 1 0 1
1 0 1 0 0 1 1 1 0 1

Sample Outputs:
2
4
7

Solution (Python):
def find(edges, tree, result, node, par, par_flip, gp_flip, out):
    ans = 0
    flip = gp_flip
    if gp_flip and tree[node - 1] == result[node - 1] or (not gp_flip and tree[node - 1] != result[node - 1]):
        flip = not flip
        out.append(node)
        ans += 1
    for child in edges.get(node, []):
        if child != par:
            ans += find(edges, tree, result, child, node, flip, par_flip, out)
    return ans

def main():
    from collections import defaultdict
    n = int(input())
    edges = defaultdict(list)
    for _ in range(n - 1):
        (x, y) = list(map(int, input().split()))
        edges[x].append(y)
        edges[y].append(x)
    tree = list(map(int, input().split()))
    result = list(map(int, input().split()))
    out = []
    print(find(edges, tree, result, node=1, par=None, par_flip=False, gp_flip=False, out=out))
    print('\n'.join(map(str, out)))
if __name__ == '__main__':
    import sys
    import threading
    sys.setrecursionlimit(10 ** 6)
    threading.stack_size(10 ** 8)
    t = threading.Thread(target=main)
    t.start()
    t.join()

Solution (Python):
N = int(100000.0 + 3)
n = int(input())
adj = list(([] for i in range(N)))
for _ in range(n - 1):
    (u, v) = map(int, input().split())
    adj[u].append(v)
    adj[v].append(u)
a = [0] + list(map(int, input().split()))
b = [0] + list(map(int, input().split()))

def dfs(u, p, c_lvl, p_lvl, d):
    stk = [(u, p, c_lvl, p_lvl)]
    while stk:
        (u, p, c_lvl, p_lvl) = stk.pop()
        if c_lvl != d[u]:
            c_lvl = 1 - c_lvl
            res.append(str(u))
        for v in adj[u]:
            if v != p:
                stk += [(v, u, p_lvl, c_lvl)]
d = [i ^ j for (i, j) in zip(a, b)]
res = []
dfs(1, 0, 0, 0, d)
print(len(res))
print('\n'.join(res))

Solution (Python):
import sys
import threading
for test in range(1):
    n = int(input())
    global adj
    adj = list(([] for i in range(n + 1)))
    for i in range(n - 1):
        (a, b) = map(int, input().split())
        adj[a].append(b)
        adj[b].append(a)
    cur = [0] + list(map(int, input().split()))
    tobe = [0] + list(map(int, input().split()))
    ans = 0
    op = []

    def dfs(node, par=-1, par_changes=0, gpar_changes=0):
        if gpar_changes % 2:
            cur[node] ^= 1
        if cur[node] != tobe[node]:
            cur[node] ^= 1
            global ans
            ans += 1
            global op
            op.append(node)
            gpar_changes += 1
        for child in adj[node]:
            if child == par:
                continue
            dfs(child, node, gpar_changes, par_changes)

def main():
    dfs(1)
    print(ans)
    for i in op:
        print(i)
if __name__ == '__main__':
    sys.setrecursionlimit(10 ** 6)
    threading.stack_size(10 ** 8)
    t = threading.Thread(target=main)
    t.start()
    t.join()