PreInPostTraversal.md

Pre-Order Traversal

/**
 * 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;
 *     }
 * }
 */

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        
        Stack<TreeNode> stack = new Stack<>();
        List<Integer> result = new ArrayList<>();


        while(root!=null || stack.size()>0){

            while(root!=null){
                result.add(root.val);
                stack.push(root);
                root = root.left;
            }

            root = stack.pop();

            if(root!=null){
                root = root.right;
            }

        }

        return result;
    }
}

In-Order Traversal

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        
        List<Integer> result = new ArrayList<>();
        if(root==null){
            return result;
        }
        Stack<TreeNode> stack = new Stack<>();

        while(root!=null || stack.size()>0){

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

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

            root = root.right;
        }

        return result;
    }
}

Post-Order Traversal

• LRN
  • NRL -> and reverse the output array

Note - if i use, LinkedList instead of ArrayList, then i can reduce so much run time of the program.

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

        while(root!=null || stack.size()>0){

            while(root!=null){
                //result.add(root.val);
                result.addFirst(root.val); // Reverse the process of preorder
                stack.push(root); 
                root = root.right; // Reverse the process of preorder
            }

            root = stack.pop();

            if(root!=null){
                root = root.left; // Reverse the process of preorder
            }
        }

        //Collections.reverse(result);

        return result;
    }
}

Reference

Preorder, Inorder, and Postorder Iteratively Summarization