给定一个二叉树，原地将它展开为链表。

例如

给定二叉树
    1
   / \
  2   5
 / \   \
3   4   6
将其展开为：
1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

代码

class Solution {
public:
    void flatten(TreeNode *root) {
        if (!root) return;
        if (root->left) flatten(root->left);
        if (root->right) flatten(root->right);
        TreeNode *tmp = root->right;
        root->right = root->left;
        root->left = NULL;
        while (root->right) root = root->right;
        root->right = tmp;
    }
};