给定一个二叉树的根节点 root ，返回它的 中序 遍历。

示例 1：

输入：root = [1,null,2,3]
输出：[1,3,2]

/**
 * 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 ans = []

    function centerTravser(node) {
        if(node !== null) {
            centerTravser(node.left)
            ans.push(node.val)
            centerTravser(node.right)
        }
    }

    centerTravser(root)

    return ans
};

/**
 * 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 ans = []
    let stack = []
    while (root || stack.length) {
        while (root) {
            stack.push(root)
            root = root.left
        }
        root = stack.pop()
        ans.push(root.val)
        root = root.right
    }

    return ans
};

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/binary-tree-inorder-traversal/