Leetcode 623. Add One Row to Tree

题目链接：leetcode 623

Given the root of a binary tree, then value v and depth d, you need to add a row of nodes with value v at the given depth d. The root node is at depth 1.

　　题目很简单，在树的第d层加一层，值为v。递归增加一层就好了。代码如下
　

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public TreeNode addOneRow(TreeNode root, int v, int d) {        if (null == root)            return null;        if (1 == d) {            TreeNode left = new TreeNode(v);            left.left = root;            root = left;        }        else if (2 == d) {            TreeNode left = new TreeNode(v);            TreeNode right = new TreeNode(v);            left.left = root.left;            right.right = root.right;            root.left = left;            root.right = right;        }        else {            addOneRow(root.left,  v, d-1);            addOneRow(root.right, v, d-1);        }        return root;    }}