LeetCode_589

先看题目

 

前序遍历N叉树，与前序遍历二叉树类似

可以使用递归或者栈，两种方法

1.使用递归

/*
// 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> res = new LinkedList<>();
        pre(res,root);
        return res;
    }
    public void pre(List<Integer> res,Node root){
        if(root == null){
            return;
        }
        res.add(root.val);
        for(Node node : root.children){
            pre(res,node);
        }
    }
}

2.使用栈

/*
// 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> res = new LinkedList<>();
        if(root == null){
            return res;
        }
        Stack<Node> stack1 = new Stack<>();
        Stack<Node> stack2 = new Stack<>();
        
        stack2.add(root);
        while(stack2.size() > 0){
            Node node = stack2.pop();
            res.add(node.val);
            for(Node n : node.children){
                stack1.add(n);
            }
            while(stack1.size() > 0){
                stack2.add(stack1.pop());
            }
        }
        return res;
    }
}