FG面经Prepare: BST to Double LinkedList

BST to double linkedlist in inorder traversal sequence
Follow Up: 如果这个BST自带prev, next, 给一个value，插进去这个node，并补全left，right，prev，next

 class TreeNode {
  public int val;
  TreeNode left;
  TreeNode right;
 }

 TreeNode newRoot

 TreeNode tree2Dll(TreeNode root) {
   TreeNode pre = null;
   TreeNode newRoot = null;
   helper(root, pre);
   return newRoot;
 }

 public void helper(TreeNode cur, TreeNode pre) {
   if (cur == null) {
     return;
   }
   helper(cur.left, pre);
   cur.left = pre;
   if (pre == null) {
      newRoot = cur;
   }
   else pre.right = cur;
   pre = cur;
   helper(cur.right, pre, newRoot);
 }

Follow up:

第一步完成基础上，把这个new value插入BST中，一边插入一边记录沿途的大于value的root node（即走了该root node的left child），作为inorder successor. 找到inorder successor之后，通过其prev指针找到inorder predecessor, 改指针就好了