Google – Reset Invalid Node


Google – Reset Invalid Node
有个tree, 每个node有getId(), setId(), 还有一个Iterator来表示children。Node里面id为null或者负数都是invalid node,且id不能重复,要求找出所有Invalid node并且重新赋值

[Solution]
[Solution #1]
找max,从max + 1开始重新赋值。
cons: 会Overflow.
[Solution #2]
用HashSet找first missing number
因为每次碰到一个invalid node,都会去搜索下一个first missing number,所以时间复杂度应该还是O(n)吧。不知道对不对,感觉怪怪的,可是又想不出什么会导致n^2的case。
class Solution {
  int val = 0;
  public void resetInvalidNode(TNode root) {
    if (root == null) {
      return;
    }
 
    Set<Integer> set = new HashSet<>();
    dfs(root, set);
 
    reset(root, set);
  }
 
  private void dfs(TNode root, Set<Integer> set) {
    if (root != null && root.val >= 0) {
      set.add(root.val);
    }
 
    Iterator<TNode> it = root.iterator;
    while (it.hasNext()) {
      dfs(it.next(), set);
    }
  }
 
  private void reset(TNode root, Set<Integer> set) {
    if (root.val < 0 && root.val == null) {//???
      while (set.contains(this.val)) {
        this.val++;
      }
      root.val = val;
    }
 
    Iterator<TNode> it = root.iterator;
    while (it.hasNext()) {
      dfs(it.next(), set, this.val);
    }
  }
}
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