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 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.

On, On

/** * 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; Stack<TreeNode> stack = new Stack<>(); stack.push(root); while (!stack.isEmpty()) { TreeNode node = stack.pop(); if (node.right != null) stack.push(node.right); if (node.left != null) stack.push(node.left); if (!stack.isEmpty()) node.right = stack.peek(); node.left = null; } } }

On, O1

/** * 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; TreeNode node = root; while (node != null) { if (node.left != null) { TreeNode tmp = node.left; while (tmp.right != null) tmp = tmp.right; tmp.right = node.right; node.right = node.left; node.left = null; } node = node.right; } } }