## Flatten Binary Tree to Linked List

### 描述

Given a binary tree, flatten it to a linked list in-place.

For example, Given

         1
/ \
2   5
/ \   \
3   4   6


The flattened tree should look like:

   1
\
2
\
3
\
4
\
5
\
6


### 递归版1

// Flatten Binary Tree to Linked List
// 递归版1，时间复杂度O(n)，空间复杂度O(logn)
class Solution {
public:
void flatten(TreeNode *root) {
if (root == nullptr) return;  // 终止条件

flatten(root->left);
flatten(root->right);

if (nullptr == root->left) return;

// 三方合并，将左子树所形成的链表插入到root和root->right之间
TreeNode *p = root->left;
while(p->right) p = p->right; //寻找左链表最后一个节点
p->right = root->right;
root->right = root->left;
root->left = nullptr;
}
};

### 递归版2

// Flatten Binary Tree to Linked List
// 递归版2
// @author 王顺达(http://weibo.com/u/1234984145)
// 时间复杂度O(n)，空间复杂度O(logn)
class Solution {
public:
void flatten(TreeNode *root) {
flatten(root, NULL);
}
private:
// 把root所代表树变成链表后，tail跟在该链表后面
TreeNode *flatten(TreeNode *root, TreeNode *tail) {
if (NULL == root) return tail;

root->right = flatten(root->left, flatten(root->right, tail));
root->left = NULL;
return root;
}
};