LeetCode OJ-- Flatten Binary Tree to Linked List **

https://oj.leetcode.com/problems/flatten-binary-tree-to-linked-list/

二叉树的处理，将二叉树转换成类似一个单链表的东东，并且原地。

类似于先跟遍历的顺序。

定义函数 

TreeNode* subflatten(TreeNode *root)
并将返回值设置为，先转换出来链表的最后一个位置。

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  };
 
class Solution {
public:
    void flatten(TreeNode *root) {
        if(root == NULL)
            return;
        subflatten(root);
    }
    TreeNode* subflatten(TreeNode *root)
    {
        if(root->left == NULL && root->right == NULL)
            return root;
        if(root->left == NULL && root->right!= NULL)
            return subflatten(root->right);

        //remember root right for temporary
        TreeNode *tempr  = new TreeNode(0);
        tempr = root->right;
        
        root->right = root->left;
        root->left = NULL;
        
        TreeNode *retNode = subflatten(root->right);

        if(tempr == NULL)
            return retNode;

        retNode->right = tempr;

        TreeNode *retNode2 = subflatten(tempr);

        return retNode2;
    }
};