94. 二叉树的中序遍历（算法200题-56）

 前、中、后是指根节点的位置
 中序遍历 =》 左 中 右
/**
 * 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) {
    const list = []
    helper(root, list)
    return list
};
const helper = (node, list) => {
    if(!node) return
   helper(node.left, list)
   list.push(node.val)
   helper(node.right, list)
}

var inorderTraversal = function (root) {
      const stack = [],
        list = []
    let node = root
    while (node || stack.length) {
        if (node) {
            stack.push(node)
            node = node.left
        } else {
            node = stack.pop()
            list.push(node.val)
            node = node.right
        }
    }
    return list
};