144. Binary Tree Preorder Traversal

Intuition

This is a standard tree traversal algorithm where we visit the root, then left node and then right node.

• Inorder traversal - left root right
• Postorder traversal - left right root

Approach

• Using Stack :
• Push the root node to the stack.
• Until the stack is not empty, first store the top element of the stack to our result.

• Preorder traversal is root -> left -> right.
  Since we use stack which is LIFO, we first push the right node to the stack, then push the left node to the stack.

• Using recursion :

• Check whether the node is null and this is the base condition for recursion.

• Store the node value to our result.

• Visit the left node.

• Visit the right node.

Complexity

• Time complexity:
  We are going through all the nodes of the tree. If the tree have n number of nodes, then time complexity will be O(n).

• Space complexity:
  We store maximum number of element = height of the tree to the stack. So space complexity = O(H) where H = height of tree

Code

/**
 * 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) {
        // preorder - root left right
        // inorder - left root right
        // postorder - left right root
        List<Integer> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        byUsingStack(root, result);
        byUsingRecursion(root, result);
        return result;
    }

    private void byUsingStack(TreeNode node, List<Integer> result) {
        Stack<TreeNode> stack = new Stack<>();
        stack.push(node);
        while (!stack.isEmpty()) {
            TreeNode top = stack.pop();
            result.add(top.val);
            if (top.right != null) {
                stack.push(top.right);
            }
            if (top.left != null) {
                stack.push(top.left);
            }
        }
    }

    private void byUsingRecursion(TreeNode node, List<Integer> result) {
        if (node == null) {
            return;
        }
        result.add(node.val);
        byUsingRecursion(node.left, result);
        byUsingRecursion(node.right, result);
    }
}