285. Inorder Successor in BST

 1     public TreeNode inorderSuccessor(TreeNode root, TreeNode p) {
 2         if(root == null) {
 3             return null;
 4         }
 5         Stack<TreeNode> stack = new Stack<TreeNode>();
 6         TreeNode cur = root;
 7         TreeNode pre = null;
 8         while(cur != null || !stack.isEmpty()) {
 9             while(cur != null) {
10                 stack.push(cur);
11                 cur = cur.left;
12             }
13             cur = stack.pop();
14             if(pre != null && pre == p) {
15                 return cur;
16             }
17             pre = cur;
18             cur = cur.right;
19         }
20         return null;
21     }