LeetCode-114 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

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

 

分析：

使用递归，存储原始树的左子树。需要注意的是题目要求in-place,因此不能用new TreeNode(X)构造新节点并作为结果返回。

 

代码如下：

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}

public class Solution {
    public void flatten(TreeNode root) {
        if(root != null) {
            TreeNode left = root.left;
            TreeNode right = root.right;
            
            root.left = null;
            root.right = left;
            flatten(root.right);
            
            while(root.right != null) {
                root = root.right;
            }
            
            root.right = right;
            flatten(root.right);
        }
    }
}