leetcode讲解--589. N-ary Tree Preorder Traversal

题目

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

For example, given a 3-ary tree:

Return its preorder traversal as: [1,3,5,6,2,4].

Note:

Recursive solution is trivial, could you do it iteratively?

题目地址

讲解

如果用递归来解题的话，还是非常简单的。如果要用迭代来解题，无非就是用栈来实现。要注意的一点是，需要一个额外的栈来把压栈的顺序倒一下序。

Java代码

递归代码：

/*
// Definition for a Node.
class Node {
    public int val;
    public List children;

    public Node() {}

    public Node(int _val,List _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
    private List result = new ArrayList<>();
    public List preorder(Node root) {
        if(root==null){
            return result;
        }
        result.add(root.val);
        for(Node node:root.children){
            preorder(node);
        }
        return result;
    }
}

迭代代码：

/*
// Definition for a Node.
class Node {
    public int val;
    public List children;

    public Node() {}

    public Node(int _val,List _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
    private List result = new ArrayList<>();
    private Stack stack = new Stack<>();
    private Stack stack_temp = new Stack();
    public List preorder(Node root) {
        if(root==null){
            return result;
        }
        stack.push(root);
        while(!stack.empty()){
            Node theNode = stack.pop();
            result.add(theNode.val);
            for(Node node:theNode.children){
                stack_temp.push(node);
            }
            while(!stack_temp.empty()){
                stack.push(stack_temp.pop());
            }
        }
        return result;
    }
}