[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;
     }