Input:  A binary tree as following:        4     /   \     2     6   / \   /   3   1 5     v = 1  d = 2  Output:         4      / \      1   1    /     \    2       6  / \     /  3   1   5    来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/add-one-row-to-tree

/**  * @param {number} cd:current depth,递归当前深度  * @param {number} td:target depth, 目标深度  */var traversal = function (node, v, cd, td) {     // 递归到目标深度，创建新节点并返回  if (cd === td) {     // return 新节点  }   // 向左子树递归  if (node.left) {     node.left = traversal (node.left, v, cd + 1, td);   }   // 向右子树递归  if (node.right) {     node.right = traversal (node.right, v, cd + 1, td);   }   // 返回旧节点  return node; }; /**  * Definition for a binary tree node.  * function TreeNode(val) {  *     this.val = val;  *     this.left = this.right = null;  * }  *//**  * @param {TreeNode} root  * @param {number} v  * @param {number} d  * @return {TreeNode}  */var addOneRow = function (root, v, td) {     // 从根节点开始递归  traversal (root, v, 1, td);   return root; };