leetcode-623. Add One Row to Tree

考察点：递归， 二叉树；
思路：递归思想；很重要的是help函数的处理，它要返回指针，而且它的功能是返回最终的root，这个root是满足条件的root。当然，help函数里还要递归，确定好为NULL的条件和depth==curdepth这两个结束条件就ok了。
C++代码：

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    TreeNode* addOneRow(TreeNode* root, int v, int d) {        if (d == 1) {            TreeNode * ret = new TreeNode(v);            ret->left = root;            return ret;        }         return addOneRow(root, v, d, 1);    }    TreeNode* addOneRow(TreeNode* root, int v, int d, int curDepth) {        if (root == NULL)            return NULL;        if (d == curDepth + 1) {            TreeNode* tempLeft = root->left;            TreeNode* tempRight = root->right;            TreeNode * newLeft = new TreeNode(v);            TreeNode * newRight = new TreeNode(v);            root->left = newLeft;            root->right = newRight;            newLeft->left = tempLeft;            newRight->right = tempRight;            return root;        } else {            addOneRow(root->left, v, d, curDepth+1);            addOneRow(root->right, v, d, curDepth+1);        }        return root;    }};