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