/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode *father;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL), father(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* inorderSuccessor(TreeNode* p) {
        if (p -> right) {
            p = p -> right;
            while (p -> left) p = p -> left;
            return p;
        }

        while (p -> father && p == p -> father -> right) p = p -> father;

        return p -> father;
    }
};