/**
 * 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) return nullptr;
        if(p->right) {
            p = p->right;
            while(p->left) p = p->left;
            return p;
        }
        while(p->father && p->father->right == p) p = p->father;
        if(p->father) return p->father;
        return nullptr;
    }
};