590. N-ary Tree Postorder Traversal

Problem

Given an n-ary tree, return the postorder traversal of its nodes' values.For example, given a 3-ary tree:Return its postorder traversal as: [5,6,3,2,4,1].Note: Recursive solution is trivial, could you do it iteratively?

Solution (Recursion)

class Solution {
    public List<Integer> postorder(Node root) {
        List<Integer> res = new ArrayList<>();
        helper(root, res);
        return res;
    }
    private void helper(Node root, List<Integer> res) {
        if (root == null) return;
        if (root.children != null) {
            for (Node child: root.children) {
                helper(child, res);
            }
        }
        res.add(root.val);
    }
}
 

Solution (Iteration)

按順序放入stack，正好level方面是從root到leaves，順序方面是從最右到最左，因為stack是先入後出。這樣最後reverse一下就是先左後右，先子後根。巧妙。

class Solution {
    public List<Integer> postorder(Node root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) return res;
        Stack<Node> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            Node node = stack.pop();
            res.add(node.val);
            for (Node child: node.children) {
                stack.push(child);
            }
        }
        Collections.reverse(res);
        return res;
    }
}
 

145. Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]

   1
    \
     2
    /
   3

Output: [3,2,1]Follow up: Recursive solution is trivial, could you do it iteratively?

Solution (Iteration)

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) return res;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            res.add(node.val);
            if (node.left != null) stack.push(node.left);
            if (node.right != null) stack.push(node.right);
        }
        Collections.reverse(res);
        return res;
    }
}
 

Solution (Recursion)

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        helper(root, res);
        return res;
    }
    private void helper(TreeNode node, List<Integer> res) {
        if (node == null) return;
        if (node.left != null) helper(node.left, res);
        if (node.right != null) helper(node.right, res);
        res.add(node.val);
    }
}