leetcode--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.

method 1 recursive method:

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

　　

method2: preorder tree traversal method

public class Solution {
    public void flatten(TreeNode root) {
        Stack<TreeNode> nodes = new Stack<TreeNode>();
        if(root != null){
		    TreeNode currentNode = root;
			if(root.right != null)
			    nodes.push(root.right);
			if(root.left != null){
				nodes.push(root.left);
				root.left = null;
			}
			while(!nodes.isEmpty()){
				TreeNode aNode = nodes.pop();
				currentNode.right = aNode;
				//currentNode.left = null;
				currentNode = currentNode.right;
				if(aNode.right != null)
					nodes.push(aNode.right);
				if(aNode.left != null){
					nodes.push(aNode.left);
					aNode.left = null;
				}
			}
        }
    }
}