[leetcode] 114. Flatten Binary Tree to Linked List

题目：

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

click to show hints.

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题解：

首先hint提示了类似前序遍历。所以只要将前序遍历的结果保存到一颗只有右子树，左子树为null的二叉树里就行了。

preorder函数：

先判断root是否为空，然后保存左右子树分别为left和r。再根据left和r的值重构root指向的二叉树。

代码：

class Solution {
public:
TreeNode * preorder(TreeNode *root)
{
if(!root)
return root;
TreeNode *left=root->left;
TreeNode *r=root->right;
root->left=NULL;
if(left)
{
root->right=left;

root= preorder(left);
}
if(r)
{
root->right=r;
root=preorder(r);
}
return root;
}
void flatten(TreeNode* root) {
if(!root)
return ;
preorder(root);


}
};