145. Binary Tree Postorder Traversal

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LeetCode

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Problem

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

Example 1:

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

Example 2:

Input: root = []
Output: []

Example 3:

Input: root = [1]
Output: [1]

Code

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

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 cur = stack.pop();
            res.add(0, cur.val);
            if(cur.left != null) stack.push(cur.left);
            if(cur.right != null) stack.push(cur.right);
        }
        
        return res;
    }
}