LeetCode 144. Binary Tree Preorder Traversal

Description

https://leetcode.com/problems/binary-tree-preorder-traversal/description/

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

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

Explanation

To return the preorder traversal of tree nodes values iteratively, we introduce a stack. With a stack, we can easily iterative nodes with preorder (root, left, right) sequence.

Video Tutorial

Java Solution

/**
 * 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> result = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        
        if (root == null) {
            return result;
        }
        
        stack.push(root);
        while (!stack.empty()) {
            TreeNode node = stack.pop();
            result.add(node.val);
            
            if (node.right != null) {
                stack.push(node.right);
            }
            if (node.left != null) {
                stack.push(node.left);
            }
        }
        
        return result;
    }
}

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