算法: 多叉树前序遍历N-ary Tree Preorder Traversal

Posted 2021-09-14 架构师易筋

N-ary Tree Preorder Traversal

Given the root of an n-ary tree, return the preorder traversal of its nodes’ values.

Nary-Tree input serialization is represented in their level order traversal. Each group of children is separated by the null value (See examples)

Example 1:

Input: root = [1,null,3,2,4,null,5,6]
Output: [1,3,5,6,2,4]

Example 2:

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: [1,2,3,6,7,11,14,4,8,12,5,9,13,10]

Constraints:

The number of nodes in the tree is in the range [0, 104].
0 <= Node.val <= 104
The height of the n-ary tree is less than or equal to 1000.
 

Follow up: Recursive solution is trivial, could you do it iteratively?

递归解法

所有树的前序、中序、后序解法都是一个模板，不同仅仅在于list.add(item) 的位置，是在前面，中间，还是后面。

/*
// 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> preorder(Node root) {
        List<Integer> list = new ArrayList<Integer>();
        helper(root, list);
        return list;
    }
    
    private void helper(Node root, List<Integer> list) {
        if (root == null) return;
        
        list.add(root.val);
        for (Node node: root.children) {
            helper(node, list);
        }
    }
}