LeetCode114 Flatten Binary Tree to Linked List

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

For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6

The flattened tree should look like:

   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

分析：

将树的问题和链表插入问题结合。对于每个节点，寻找左子树的最右端，它应该是左子树前序遍历的最后一位，也就是生成链表中，右子树根节点的前一位。

按照链表插入的方法处理即可。

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:
     void flatten(TreeNode* root) {
         if (root == nullptr) {
             return;
         }
         while (root != nullptr) {
             if (root -> left != nullptr) {
                 TreeNode* temp = root -> left;
                 while (temp -> right != nullptr) {
                     temp = temp -> right;
                 }
                 temp -> right = root -> right;
                 root -> right = root -> left;
                 root -> left = nullptr;
             }
             root = root -> right;
         }
     }
 };