After the tree traversal sequence Leetcode590.N

Title Description

Given an N-ary tree, whose nodes after the sequence returns from traversing.

Example:

Returns thereafter preorder: [5,6,3,2,4,1].

answer

After the first interpolation preorder (java)

Ideas: The tree is traversed, at the same time to ensure that the head plug preorder. (+ Stack head plug)

/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> children;
    public Node() {}
    public Node(int _val) {
        val = _val;
    }
    public Node(int _val, List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
    public List<Integer> postorder(Node root) {
        LinkedList<Node> stack = new LinkedList<>();
        LinkedList<Integer> output = new LinkedList<>();
        if (root == null) {
            return output;
        }
      stack.add(root);
      while (!stack.isEmpty()) {
          Node node = stack.pollLast();
          output.addFirst(node.val);
          for (Node item : node.children) {
              if (item != null) {
                  stack.add(item);    
              } 
          }
      }
      return output;
    }
}

Complexity Analysis

• time complexity: $O (N)$
• Space complexity: $O (N)$

Recursive (java)

Ideas:

class Solution {
    public List<Integer> postorder(Node root) {
        List<Integer> res  =  new ArrayList<Integer>();
        if(root == null) return res;
        for(Node cur:root.children){
            res.addAll(postorder(cur));
        }
        res.add(root.val);
        return res;
    }
}

Complexity Analysis

• time complexity: $O (N)$
• Space complexity: $O (N)$