[LC] 114. Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example, given the following tree:

    1
   / 
  2   5
 /    
3   4   6

The flattened tree should look like:

1
 
  2
   
    3
     
      4
       
        5
         
          6

Solution 1:

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def flatten(self, root):
        """
        :type root: TreeNode
        :rtype: None Do not return anything, modify root in-place instead.
        """
        self.prev = None
        def dfs(root):
            if root is None:
                return None
            # reverse preOrder traveral
            # use pre_node to track the previous node to connect as current right
            dfs(root.right)
            dfs(root.left)
            root.right = self.prev
            root.left = None
            self.prev = root
        dfs(root)

Solution 2:

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

Solution 3:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public void flatten(TreeNode root) {
        if (root == null) {
            return;
        }
        LinkedList<TreeNode> stack = new LinkedList<>();
        stack.offerFirst(root);
        while (!stack.isEmpty()) {
            TreeNode cur = stack.pollFirst();
            if (cur.right != null) {
                stack.offerFirst(cur.right);
            }
            if (cur.left != null) {
                stack.offerFirst(cur.left);
            }
            if (!stack.isEmpty()) {
                cur.right = stack.peekFirst();
            }
            cur.left = null;
        }
    }
}