144. Binary Tree Preorder Traversal

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

JavaScript

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var preorderTraversal = function(root) {
  if (!root) {
    return []
  }

  var result = [root.val]
  result = result.concat(preorderTraversal(root.left))
  result = result.concat(preorderTraversal(root.right))
  return result
}

JavaScript

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var preorderTraversal = function(root) {
  if (!root) {
    return []
  }

  var result = []
  var stack = [root]

  while (stack.length) {
    let node = stack.pop()
    result.push(node.val)
    if (node.right) {
      stack.push(node.right)
    }
    if (node.left) {
      stack.push(node.left)
    }
  }

  return result
}

Python

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None


class Solution(object):

    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        result = [root.val]
        result.extend(self.preorderTraversal(root.left))
        result.extend(self.preorderTraversal(root.right))
        return result

Python

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None


class Solution(object):

    def preorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        if not root:
            return []
        result = []
        stack = [root]
        while stack:
            node = stack.pop()
            result.append(node.val)
            if node.right:
                stack.append(node.right)
            if node.left:
                stack.append(node.left)
        return result