class Solution {
public:
    vector<TreeNode*> record;
    void DFS(TreeNode* p){
        if(!p) return;

        DFS(p->left);
        record.push_back(p);
        DFS(p->right);
    }
    TreeNode* inorderSuccessor(TreeNode* p) {
        if(!p) return NULL;
        auto cur = p;
        while(p->father) p = p->father;
        DFS(p);
        int i = 0;
        for(;i < record.size();i++){
            if(record[i] == cur){
                break;    
            }
        }
        return (i < record.size() - 1)?record[i+1]:NULL;
    }
};