Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 * int val;
 * TreeNode left;
 * TreeNode right;
 * TreeNode() {}
 * TreeNode(int val) { this.val = val; }
 * TreeNode(int val, TreeNode left, TreeNode right) {
 * this.val = val;
 * this.left = left;
 * this.right = right;
 * }
 * }
 */

// Recursive way

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        postOrder(root, result);
        return result;

    }

    public void postOrder(TreeNode root, List<Integer> result) {
        if (root == null) {
            return;
        }

        postOrder(root.left, result);
        postOrder(root.right, result);
        result.add(root.val);
    }

}

// Iterative approach
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        TreeNode current = root;
        Stack<TreeNode> stack = new Stack<>();

        while (current != null || !stack.isEmpty()) {
            if (current != null) {
                stack.push(current);
                current = current.left;
            } else {

                TreeNode temp = stack.peek().right;
                if (temp == null) {

                    temp = stack.pop();
                    result.add(temp.val);

                    while (!stack.isEmpty() && temp == stack.peek().right) {
                        temp = stack.pop();
                        result.add(temp.val);
                    }
                } else {
                    current = temp;
                }
            }
        }
        return result;
    }
}