Binary Tree Preorder Traversal - The Coding Shala

Last Updated: 19-Jan-2021

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 In this post, we will learn how to Traverse a Binary Tree in Pre-Order.

Binary Tree Preorder Traversal

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

Example:

Input: [1,null,2,3]

   1
    \
     2
    /
   3

Output: [1,2,3]

Preorder Traversal of Binary Tree in Java

Approach 1

Using Recursion.

Java Program: 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> Pre_Order = new ArrayList<Integer>();
        if(root == null) return Pre_Order;
        Pre_Order.add(root.val);
        Pre_Order.addAll(preorderTraversal(root.left));
        Pre_Order.addAll(preorderTraversal(root.right));
        return Pre_Order;
    }
}

Approach 2

Iterative solution. Using Stack[DFS].

Java Program: 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<Integer>();
        if(root == null) return ans;
        Stack<TreeNode> stack = new Stack<>();
        stack.add(root);
        while(!stack.empty()){
            TreeNode curr = stack.pop();
            ans.add(curr.val);
            if(curr.right != null) stack.push(curr.right);
            if(curr.left != null) stack.push(curr.left);
        }
        return ans;
    }
}

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• Introduction to Binary Tree
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• Introduction to Dynamic Array

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