leetcode-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

class Solution {
public:
    void flatten(TreeNode* root) {
        
        if(root == nullptr) {
            return;
        }
        
        helper(root);
    }
private:
    TreeNode* pre = nullptr;
    void helper(TreeNode* root) {
        
        if(root != nullptr) { 
            helper(root->right);
            helper(root->left);
            root->right = pre;
            root->left = nullptr;
            pre = root;
        }
    }
};

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