leetcode 590. N叉樹的後序遍歷

給定一個 N 叉樹，返回其節點值的後序遍歷。

例如，給定一個 3叉樹 :

返回其後序遍歷: [5,6,3,2,4,1].

說明: 遞歸法很簡單，你可以使用迭代法完成此題嗎?

迭代法

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

    public Node() {}

    public Node(int _val,List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
    public List<Integer> postorder(Node root) {
        List<Integer> result = new ArrayList<>();
        if(root == null){
            return result;
        }
        Stack<Node> stack = new Stack<>();
        stack.push(root);
        Node pre = null;
        while(stack.size() > 0){
            root = stack.peek();
            if(root.children.size()==0||(pre != null&&root.children.contains(pre))){
                result.add(root.val);
                pre = stack.pop();
            }else{
                for(int i = root.children.size() - 1;i >= 0;i--){
                    stack.add(root.children.get(i));
                }
            }
        }
        return result;
    }
}

遞歸法

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

    public Node() {}

    public Node(int _val,List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
    
    List<Integer> list = new ArrayList<>();
    public List<Integer> postorder(Node root) {
        getResult(root);
        return list;
    }
    
    public void getResult(Node root){
        if(root == null){
            return;
        }
        for(int i = 0;i < root.children.size();i++){
            getResult(root.children.get(i));
        }
        list.add(root.val);
    }
}