結果

問題 No.1928 Make a Binary Tree
コンテスト
ユーザー gew1fw
提出日時 2025-06-12 15:03:52
言語 PyPy3
(7.3.15)
結果
WA  
実行時間 -
コード長 2,634 bytes
コンパイル時間 465 ms
コンパイル使用メモリ 82,376 KB
実行使用メモリ 849,348 KB
最終ジャッジ日時 2025-06-12 15:05:01
合計ジャッジ時間 31,239 ms
ジャッジサーバーID
（参考情報） 		judge4 / judge2
このコードへのチャレンジ
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ファイルパターン 結果
sample AC * 3
other AC * 3 WA * 32 MLE * 12 -- * 10
権限があれば一括ダウンロードができます

ソースコード

diff # 

import sys
from collections import deque

def main():
    sys.setrecursionlimit(1 << 25)
    N = int(sys.stdin.readline())
    adj = [[] for _ in range(N + 1)]
    for _ in range(N - 1):
        x, y = map(int, sys.stdin.readline().split())
        adj[x].append(y)
        adj[y].append(x)
    
    # Build the tree structure with parent and children
    parent = [0] * (N + 1)
    children = [[] for _ in range(N + 1)]
    visited = [False] * (N + 1)
    q = deque([1])
    visited[1] = True
    while q:
        u = q.popleft()
        for v in adj[u]:
            if not visited[v]:
                visited[v] = True
                parent[v] = u
                children[u].append(v)
                q.append(v)
    
    # Iterative post-order traversal
    post_order = []
    stack = [(1, False)]
    while stack:
        node, processed = stack.pop()
        if processed:
            post_order.append(node)
            continue
        stack.append((node, True))
        # Push children in reverse order to process left to right
        for child in reversed(children[node]):
            stack.append((child, False))
    
    dp = [0] * (N + 1)
    candidates = [ (0, 0) for _ in range(N + 1) ]  # Each node's top two candidates
    
    for u in post_order:
        max1 = -float('inf')
        max2 = -float('inf')
        for v in children[u]:
            c1, c2 = candidates[v]
            # Update max1 and max2 with c1
            if c1 > max1:
                max2 = max1
                max1 = c1
            elif c1 > max2:
                max2 = c1
            # Update max1 and max2 with c2
            if c2 > max1:
                max2 = max1
                max1 = c2
            elif c2 > max2:
                max2 = c2
        
        # Compute sum_candidates
        if max1 == -float('inf'):
            sum_candidates = 0
        elif max2 == -float('inf'):
            sum_candidates = max1
        else:
            sum_candidates = max1 + max2
        dp[u] = 1 + sum_candidates
        
        # Determine the new candidates for u
        a = max1 if max1 != -float('inf') else -float('inf')
        b = max2 if max2 != -float('inf') else -float('inf')
        c = dp[u]
        new_candidates = [a, b, c]
        new_candidates.sort(reverse=True)
        # Take the first two, replace -inf with -inf
        c1 = new_candidates[0] if new_candidates[0] != -float('inf') else -float('inf')
        c2 = new_candidates[1] if len(new_candidates) > 1 and new_candidates[1] != -float('inf') else -float('inf')
        candidates[u] = (c1, c2)
    
    print(dp[1])

if __name__ == '__main__':
    main()
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