二叉搜索树建双向链表

链接：https://www.nowcoder.com/questionTerminal/947f6eb80d944a84850b0538bf0ec3a5
来源：牛客网

public TreeNode ConvertBSTToBiList(TreeNode root) {
        if(root==null)
            return null;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode p = root;
        TreeNode pre = null;// 用于保存中序遍历序列的上一节点
        boolean isFirst = true;
        while(p != null|| !stack.isEmpty()) {
            while (p != null){
                stack.push(p);
                p = p.left;
            }
            p = stack.pop();
            if (isFirst) {
                root = p;// 将中序遍历序列中的第一个节点记为root
                pre = root;
                isFirst = false;
            } else {
                pre.right = p;
                p.left = pre;
                pre = p;
            }      
            p = p.right;
        }
        return root;
    }