/**
 * 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){
            TreeNode* curr = p->right;
            while (curr->left)
                curr = curr->left;
            return curr;
        }

        TreeNode* curr = p;
        TreeNode* father = p->father;
        while(father && curr == father->right){
            curr = father;
            father = father->father;
        }
        return father;
    }
};

要考虑多种情况