582. Kill Process

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题目其实给了hint，说类似tree结构，那其实就是从给出的kill开始往下做 traversal，唯一需要解决的问题是 parent/children 关系的查询，直接先hash map一下pid/ppid解决，然后DFS。

class Solution:
    def killProcess(self, pid: List[int], ppid: List[int], kill: int) -> List[int]:
        children = collections.defaultdict(list)
        for i, p in enumerate(ppid):
            children[p].append(pid[i])
            
        res, stack = [], [kill]
        while stack:
            p = stack.pop()
            res.append(p)
            if p in children:
                stack.extend(children[p])
        
        return res