144. Binary Tree Preorder Traversal

Problem

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]

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

Solution 1

/**
 * 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) {
  var res = [];
  helper(root, res);
  return res;
};

var helper = function (root, res) {
  if (!root) return;
  res.push(root.val);
  helper(root.left, res);
  helper(root.right, res);
};

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).

Solution 2

/**
 * 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 res = [];
  var queue = [root];
  var node = null;
  while (queue.length) {
    node = queue.pop();
    res.push(node.val);
    if (node.right) queue.push(node.right);
    if (node.left) queue.push(node.left);
  }
  return res;
};

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).