剑指Offer-26 二叉搜索树与双向列表

输入一棵二叉搜索树，将该二叉搜索树转换成一个排序的双向链表。要求不能创建任何新的结点，只能调整树中结点指针的指向。

1. 中序遍历
2. 用before指针记录上一个被访问的节点

public class Solution {
    public TreeNode Convert(TreeNode pRootOfTree) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode index = pRootOfTree;
        TreeNode before = null;
        TreeNode root = null;
        while(!stack.isEmpty()||index!=null){
            while(index != null){
                stack.push(index);
                index = index.left;
            }
            if(!stack.isEmpty()){
                index = stack.pop();
                before = modifyPointer(before,index);
                if(root == null) root = before;
                index = index.right;
            }
        }
        return root;
    }
    public TreeNode modifyPointer(TreeNode before, TreeNode index){
        if(before != null) {
            before.right = index;
            index.left = before;
        }
        return index;
    }
}