Flatten Binary Tree to Linked List

题目描述：

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

解题思路：

使用先序遍历，并利用一个额外的变量last保存遍历过的结点中的最后一个，

在先序遍历的过程中更新last即可

AC代码如下：

class Solution {
public:
void flatten(TreeNode* root) {
if (root == NULL) return;
TreeNode* last = NULL;
preorderTravel(root, last);

}

void preorderTravel(TreeNode* root, TreeNode*& last)
{
if (root == NULL) return;
TreeNode* left = root->left;
TreeNode* right = root->right;
if (last != NULL){
last->right = root;
}
root->left=NULL;
last = root;
preorderTravel(left, last);
preorderTravel(right, last);
}
};