Consider the left pointer equivalent to the prev pointer, and the right pointer equivalent to the next pointer in a linked list. Recursively link the rightmost node of the left subtree to the root, and link the root to the leftmost node of the right subtree. public static TreeNode[] BSTtoDLL (TreeNode root) { if(root == null || root.isLeaf()) return null; TreeNode[] prev = BSTtoDLL(root.left); TreeNode[] next = BSTtoDLL(root.right); if(prev != null) { prev[1].right = root; root.left = prev[1]; } if(next != null) { next[0].left = root; root.right = next[0]; } return new TreeNode[] {prev[0], next[1]}; }