## 中序遍历二叉树 * 递归 1. 递归的参数和返回值 2. 递归的终止条件 3. 单层递归里的逻辑 * 迭代 ``` /* * Definition for a binary tree node. * function TreeNode(val, left, right) { * this.val = (val===undefined ? 0 : val) * this.left = (left===undefined ? null : left) * this.right = (right===undefined ? null : right) * } */ /* @param {TreeNode} root * @return {number[]} */ var inorderTraversal = function (root) { let res = [] const inorder = function (node) { if (!node) { return } inorder(node.left) res.push(node.val) inorder(node.right) } inorder(root) return res }; ```