Flatten Binary Tree to Linked List

题目地址：

https://leetcode.com/problems/flatten-binary-tree-to-linked-list/#/description

题目：

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

解题思路：

这道题常规解法是用stack来存right child。

代码：

public void flatten(TreeNode root) {
    TreeNode curr = root;
    Stack<TreeNode> stack = new Stack<>();

    while(!stack.isEmpty() || curr != null){
        if(curr.right != null){
            stack.push(curr.right);
        }
        if(curr.left != null){
            curr.right = curr.left;
        }
        else{
            if(!stack.isEmpty()){
                curr.right = stack.pop();
            }
        }
        curr.left = null;
        curr = curr.right;
    }
}