[leetcode]Flatten Binary Tree to Linked List

Flatten Binary Tree to Linked List

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

算法思想：很明显的先序遍历。

思路：维护一个尾节点，每次递归时候，将root节点插到尾节点的右子树上，将左子树置为空，为了防止断链，要将左右子树记录下来。

代码如下：

public class Solution {
    TreeNode tail = new TreeNode(0);
    public void flatten(TreeNode root){
        if(root == null) return;
        tail.right = root;
        tail.left = null;
        tail = tail.right;
        TreeNode left = root.left;
        TreeNode right = root.right;
        if(root.left != null) flatten(left);
        if(root.right != null) flatten(right);
    }
}