[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

click to show hints.

Hints:

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

Solution: The observation is that the desired linked list is a concatenation of root, flatten(root.left), and flatten(root.right).

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/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { public void flatten(TreeNode root) { if (root == null) { return; } flatten(root.right); if (root.left == null) { return; } flatten(root.left); TreeNode p = root.left; while (p.right != null) { p = p.right; } p.right = root.right; root.right = root.left; root.left = null; return; } }