145. Binary Tree Postorder Traversal

Description

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

Example:

Input: [1,null,2,3]

image.png

Output: [3,2,1]

Follow up: Recursive solution is trivial, could you do it iteratively?

Solution

Recursion, time O(n), space O(h)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List postorderTraversal(TreeNode root) {
        List list = new ArrayList<>();
        postorderTraversalRecur(root, list);
        return list;
    }
    
    public void postorderTraversalRecur(TreeNode root, List list) {
        if (root == null) {
            return;
        }
        
        postorderTraversalRecur(root.left, list);
        postorderTraversalRecur(root.right, list);
        list.add(root.val);
    }
}

Iteration with Stack, time O(n), space O(h)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List postorderTraversal(TreeNode root) {
        List list = new LinkedList<>();
        if (root == null) {     // edge case
            return list;
        }
        
        Stack stack = new Stack<>();
        stack.push(root);
        
        while (!stack.empty()) {
            TreeNode node = stack.pop();
            list.add(node.val);
            
            if (node.left != null) {
                stack.push(node.left);
            }
            
            if (node.right != null) {
                stack.push(node.right);
            }
        }
        
        Collections.reverse(list);  // reverse in the end
        return list;
    }
}

Common solution, time O(n), space O(h)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List postorderTraversal(TreeNode root) {
        List list = new LinkedList<>();
        Stack stack = new Stack<>();
        TreeNode p = root;
        
        while (!stack.empty() || p != null) {
            if (p != null) {
                stack.push(p);
                list.add(0, p.val);     // add to the beginning before going to children
                p = p.right;            // go right
            } else {
                p = stack.pop().left;   // go left
            }
        }
        
        return list;
    }
}