Leetcode – Binary Tree Preorder Traversal (Java)

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Preorder binary tree traversal is a classic interview problem. The key to solve this problem is using a stack to store left and right children, and push right child first so that it is processed after the left child.

Java Solution

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}
 
public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> returnList = new ArrayList<Integer>();
 
        if(root == null)
            return returnList;
 
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
 
        while(!stack.empty()){
            TreeNode n = stack.pop();
            returnList.add(n.val);
 
            if(n.right != null){
                stack.push(n.right);
            }
            if(n.left != null){
                stack.push(n.left);
            }
 
        }
        return returnList;
    }
}

7 thoughts on “Leetcode – Binary Tree Preorder Traversal (Java)”

  1. @Prabhat, I understood your solution but the input seems hardcoded. How can we enter user-input for nodes with variable length of tree and display pre, post and inorder traversals.


  2. public class Solution{
    public void preOrder(TreeNode root){
    if(root==null)return;
    System.out.println(root.val);
    preOrder(root.left);
    preOrder(root.right);
    }
    }

  3. // Recursive version based on previous comments
    class Solution {
    private void preorderTraversalRec (TreeNode root, List orderedList) {
    if (root == null)
    return;
    orderedList.add(root.val);
    preorderTraversalRec(root.left, orderedList);
    preorderTraversalRec(root.right, orderedList);

    }

    public List preorderTraversal(TreeNode root) {
    List orderedList = new ArrayList();
    if (root == null) return orderedList;

    preorderTraversalRec(root, orderedList);
    return orderedList;
    }
    }

  6. //Recursive Preorder, Inorder, PostOrder traversals.

    import java.util.ArrayList;

    import java.util.List;

    public class TreeTraversal {

    static List orderedList = new ArrayList();

    public static void preOrderTraversal(TreeNode root){

    if(root == null)

    return;

    orderedList.add(root.val);

    preOrderTraversal(root.left);

    preOrderTraversal(root.right);

    }

    public static void inOrderTraversal(TreeNode root){

    if(root == null)

    return;

    inOrderTraversal(root.left);

    orderedList.add(root.val);

    inOrderTraversal(root.right);

    }

    public static void postOrderTraversal(TreeNode root){

    if(root == null)

    return;

    postOrderTraversal(root.left);

    postOrderTraversal(root.right);

    orderedList.add(root.val);

    }

    public static void main(String[] args) {

    TreeNode root = new TreeNode(2);

    TreeNode left = new TreeNode(1);

    TreeNode right = new TreeNode(3);

    root.left = left;

    root.right = right;

    System.out.print(“nPreorder traversal of 2 1 3: “);

    preOrderTraversal(root);

    for(Integer i:orderedList)

    System.out.print(i + ” “);

    orderedList.clear();

    System.out.print(“nIneorder traversal of 2 1 3: “);

    inOrderTraversal(root);

    for(Integer i:orderedList)

    System.out.print(i + ” “);

    orderedList.clear();

    System.out.print(“nPostorder traversal of 2 1 3: “);

    postOrderTraversal(root);

    for(Integer i:orderedList)

    System.out.print(i + ” “);

    }
    }

    /*
    OUTPUT:
    Preorder traversal of 2 1 3: 2 1 3
    Ineorder traversal of 2 1 3: 1 2 3
    Postorder traversal of 2 1 3: 1 3 2
    */

  7. it would be better to use recursion rather than imperative to solve binary tree traversal.
    here is my version.

    private List traversePreOrder(TreeNode root) {
    if(root == null) return Collections.EMPTY_LIST;

    List ordered = new ArrayList();

    ordered.add(root.value);
    ordered.addAll(traversePreOrder(root.left));
    ordered.addAll(traversePreOrder(root.right));

    return ordered;

    }

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