/**
 * 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;
            if (p->left == NULL && p->father->left == p) {
                p = p->father;
                return p;
            }
            while (p->left) p = p->left;
            return p;
        }
        else {
            while (p->father != NULL && p->father->left != p) p = p->father;
            if (p->father == NULL) return NULL;
            if (p->father->left == p) return p->father;
       }
    }
};