[leetcode]_Flatten Binary Tree to Linked List

题目：将一棵二叉树履平成一个类似Linked-list的东西。

思路：该过程类似于二叉树的前序遍历，但是遍历代码，我处理不来参数的变化。没AC。

-------->写的很好的解题博客

参考上述博客，思路很清楚的写出代码：

 public void flatten(TreeNode root) {
        if(root == null) return;
        
        if(root.left != null){
            TreeNode leftNode = root.left;
            TreeNode rightNode = root.right;
            root.left = null;
            root.right = leftNode;
            
            while(leftNode.right != null) leftNode = leftNode.right;
            leftNode.right = rightNode;
        }
        
        flatten(root.right);
        
16  }

采用前序遍历错误的代码：留在下方，回头来改正：

   public void flatten(TreeNode root) {
        if(root == null) return;
        
        TreeNode newRoot = null , head = null;
        Stack<TreeNode> s = new Stack<TreeNode>();
        
        while(root != null || !s.isEmpty()){
            while(root != null){
                
                //visit node
                if(newRoot == null) {
                    newRoot = new TreeNode(root.val);
                    head = newRoot;
                }else{
                    newRoot.right = new TreeNode(root.val);
                    newRoot = newRoot.right;
                }
                
                s.push(root);
                
                //visit leftChild
                root = root.left;
            }
            if(!s.isEmpty()){
                
                //visit rightChild
                root = s.pop();
                root = root.right;
            }
        }
        root = head;
    }