Binary Tree Postorder Traversal Solutions in Java

/// Source : https://leetcode.com/problems/binary-tree-postorder-traversal/description/
/// Author : liuyubobobo
/// Time   : 2017-11-17

import java.util.ArrayList;
import java.util.List;

// Recursive
// Time Complexity: O(n), n is the node number in the tree
// Space Complexity: O(h), h is the height of the tree
public class Solution1 {

    public List<Integer> postorderTraversal(TreeNode root) {

        ArrayList<Integer> res = new ArrayList<Integer>();
        postorderTraversal(root, res);
        return res;
    }

    private void postorderTraversal(TreeNode node, List<Integer> list){
        if(node != null){
            postorderTraversal(node.left, list);
            postorderTraversal(node.right, list);
            list.add(node.val);
        }
    }
}

/// Source : https://leetcode.com/problems/binary-tree-postorder-traversal/description/
/// Author : liuyubobobo
/// Time   : 2017-11-17

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

// My Non-Recursive
// Time Complexity: O(n), n is the node number in the tree
// Space Complexity: O(h), h is the height of the tree
public class Solution2 {

    private class Command{
        String s;   // go, print
        TreeNode node;
        Command(String s, TreeNode node){
            this.s = s;
            this.node = node;
        }
    };

    public List<Integer> postorderTraversal(TreeNode root) {

        ArrayList<Integer> res = new ArrayList<Integer>();
        if(root == null)
            return res;

        Stack<Command> stack = new Stack<Command>();
        stack.push(new Command("go", root));
        while(!stack.empty()){
            Command command = stack.pop();

            if(command.s.equals("print"))
                res.add(command.node.val);
            else{
                assert command.s.equals("go");
                stack.push(new Command("print", command.node));
                if(command.node.right != null)
                    stack.push(new Command("go",command.node.right));
                if(command.node.left != null)
                    stack.push(new Command("go",command.node.left));
            }
        }
        return res;
    }

}

/// Source : https://leetcode.com/problems/binary-tree-postorder-traversal/description/
/// Author : liuyubobobo
/// Time   : 2018-05-30

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

// Non-Recursive
// Using a tag to record whether the node has been visited
//
// Time Complexity: O(n), n is the node number in the tree
// Space Complexity: O(h), h is the height of the tree
public class Solution3 {

    private class TagNode{
        TreeNode node;
        boolean isFirst;
        TagNode(TreeNode node){
            this.node = node;
            this.isFirst = false;
        }
    };

    public List<Integer> postorderTraversal(TreeNode root) {

        ArrayList<Integer> res = new ArrayList<Integer>();
        if(root == null)
            return res;

        Stack<TagNode> stack = new Stack<>();
        TreeNode cur = root;
        while(cur != null || !stack.empty()){

            while(cur != null){
                stack.push(new TagNode(cur));
                cur = cur.left;
            }

            TagNode tagNode = stack.pop();
            cur = tagNode.node;
            if(tagNode.isFirst == false){
                tagNode.isFirst = true;
                stack.push(tagNode);
                cur = cur.right;
            }
            else{
                res.add(cur.val);
                cur = null;
            }
        }
        return res;
    }
}

/// Source : https://leetcode.com/problems/binary-tree-postorder-traversal/description/
/// Author : liuyubobobo
/// Time   : 2018-05-30

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

// Non-Recursive
// Using two stacks, Reverse the Preorder Traversal!
//
// Time Complexity: O(n)
// Space Complexity: O(n)
public class Solution4 {

    public List<Integer> postorderTraversal(TreeNode root) {

        ArrayList<Integer> res = new ArrayList<Integer>();
        if(root == null)
            return res;

        Stack<TreeNode> stack = new Stack<>();
        Stack<Integer> output = new Stack<>();

        stack.push(root);
        while(!stack.empty()){

            TreeNode cur = stack.pop();
            output.push(cur.val);

            if(cur.left != null)
                stack.push(cur.left);
            if(cur.right != null)
                stack.push(cur.right);
        }

        while(!output.empty())
            res.add(output.pop());
        return res;
    }
}

/// Source : https://leetcode.com/problems/binary-tree-postorder-traversal/description/
/// Author : liuyubobobo
/// Time   : 2018-05-30

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;
import java.util.LinkedList;

// Non-Recursive
// Using two stacks, Reverse the Preorder Traversal!
//
// Time Complexity: O(n)
// Space Complexity: O(n)
public class Solution5 {

    public List<Integer> postorderTraversal(TreeNode root){

        Stack<TreeNode> stack = new Stack<>();
        LinkedList<TreeNode> output = new LinkedList<>();

        TreeNode p = root;
        while(p != null || !stack.isEmpty()){
            if(p != null){
                stack.push(p);
                output.push(p);
                p = p.right;
            }
            else{
                p = stack.pop();
                p = p.left;
            }
        }

        List<Integer> res = new ArrayList<>();
        while(!output.isEmpty())
            res.add(output.pop().val);
        return res;
    }
}

/// Source : https://leetcode.com/problems/binary-tree-postorder-traversal/description/
/// Author : liuyubobobo
/// Time   : 2018-05-31

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

// Non-Recursive
// Using a pre pointer to record the last visted node
//
// Time Complexity: O(n)
// Space Complexity: O(h)
public class Solution6 {

    public List<Integer> postorderTraversal(TreeNode root) {

        ArrayList<Integer> res = new ArrayList<Integer>();
        if(root == null)
            return res;

        Stack<TreeNode> stack = new Stack<>();
        TreeNode pre = null;

        stack.push(root);
        while(!stack.empty()){

            TreeNode cur = stack.pop();
            if((cur.left == null && cur.right == null) ||
                    (pre != null && pre == cur.left && cur.right == null) ||
                    (pre != null && pre == cur.right)){
                res.add(cur.val);
                pre = cur;
            }
            else{
                stack.push(cur);
                if(cur.right != null)
                    stack.push(cur.right);
                if(cur.left != null)
                    stack.push(cur.left);
            }
        }
        return res;
    }
}

/// Source : https://leetcode.com/problems/binary-tree-postorder-traversal/description/
/// Author : liuyubobobo
/// Time   : 2018-05-31

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

// Classic Non-Recursive
// Using a pre pointer to record the last visted node
//
// Time Complexity: O(n)
// Space Complexity: O(h)
public class Solution7 {

    public List<Integer> postorderTraversal(TreeNode root) {

        ArrayList<Integer> res = new ArrayList<Integer>();
        if(root == null)
            return res;

        Stack<TreeNode> stack = new Stack<>();
        TreeNode pre = null;
        TreeNode cur = root;

        while(cur != null || !stack.empty()){

            while(cur != null){
                stack.push(cur);
                cur = cur.left;
            }

            cur = stack.pop();
            if(cur.right == null || pre == cur.right){
                res.add(cur.val);
                pre = cur;
                cur = null;
            }
            else{
                stack.push(cur);
                cur = cur.right;
            }
        }
        return res;
    }
}

/// Source : https://leetcode.com/problems/binary-tree-postorder-traversal/description/
/// Author : liuyubobobo
/// Time   : 2018-05-31

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

// Classic Non-Recursive
// Using a pre pointer to record the last visted node
//
// Time Complexity: O(n)
// Space Complexity: O(h)
public class Solution8 {

    public List<Integer> postorderTraversal(TreeNode root) {

        ArrayList<Integer> res = new ArrayList<Integer>();
        if(root == null)
            return res;

        Stack<TreeNode> stack = new Stack<>();
        TreeNode pre = null;
        TreeNode cur = root;

        while(cur != null || !stack.empty()){

            if(cur != null){
                stack.push(cur);
                cur = cur.left;
            }
            else{
                cur = stack.pop();
                if(cur.right == null || pre == cur.right){
                    res.add(cur.val);
                    pre = cur;
                    cur = null;
                }
                else{
                    stack.push(cur);
                    cur = cur.right;
                }
            }
        }
        return res;
    }
}

/// Source : https://leetcode.com/problems/binary-tree-postorder-traversal/description/
/// Author : liuyubobobo
/// Time   : 2018-05-31

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Stack;

// Morris PostOrder Traversal
//
// Time Complexity: O(n)
// Space Complexity: O(1)
public class Solution9 {

    public List<Integer> postorderTraversal(TreeNode root) {

        ArrayList<Integer> res = new ArrayList<Integer>();
        if(root == null)
            return res;

        TreeNode dummyRoot = new TreeNode(-1);
        dummyRoot.left = root;

        TreeNode cur = dummyRoot;
        while(cur != null){
            if(cur.left == null)
                cur = cur.right;
            else{
                TreeNode pre = cur.left;
                while(pre.right != null && pre.right != cur)
                    pre = pre.right;

                if(pre.right == null){
                    pre.right = cur;
                    cur = cur.left;
                }
                else{
                    pre.right = null;
                    reverseTraversal(cur.left, res);
                    cur = cur.right;
                }
            }
        }
        return res;
    }

    private void reverseTraversal(TreeNode node, ArrayList<Integer> res){
        int start = res.size();
        while(node != null){
            res.add(node.val);
            node = node.right;
        }

        int i = start, j = res.size() - 1;
        while(i < j){
            Integer t = res.get(i);
            res.set(i, res.get(j));
            res.set(j, t);

            i ++;
            j --;
        }
    }
}