Leetcode-94.js

/**
 * 94. Binary Tree Inorder Traversal
 * https://leetcode.com/problems/binary-tree-inorder-traversal/
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
// 递归 + Array
const inorderTraversal = (root, result = []) => {
 if (root) {
   inorderTraversal(root.left, result)
   result.push(root.val)
   inorderTraversal(root.right, result)
 }
 return result
}

// 纯递归
const inorderTraversal = (root) => {
  if (!root) return []  
  return inorderTraversal(root.left).concat(root.val, inorderTraversal(root.right))
}

/**
 * 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?
 */