/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode father;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode inorderSuccessor(TreeNode p) {
        if (p.right != null) {
            TreeNode node = p.right;
            while(node.left != null) {
                node = node.left;
            }
            return node;
        } else {
            while(p.father != null) {
                TreeNode father = p.father;
                if (father.left == p) {
                    return father;
                } 
                p = p.father;
            }
        }
        return null;
    }
}