94. Binary Tree Inorder Traversal

Problem

Given a binary tree, return the inorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,3,2]

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 inorderTraversal = function(root) {
  var res = [];
  helper(root, res);
  return res;
};

var helper = function (root, res) {
  if (!root) return;
  if (root.left) helper(root.left, res);
  res.push(root.val);
  if (root.right) 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 inorderTraversal = function(root) {
  var stack = [];
  var now = root;
  var res = [];

  while (now || stack.length) {
    while (now) {
      stack.push(now);
      now = now.left;
    }
    now = stack.pop();
    res.push(now.val);
    now = now.right;
  }

  return res;
};

Explain:

nope.

Complexity:

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