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class Solution {
public:
    ListNode *partition(ListNode *head, int x) {
        if(head == NULL || head->next == NULL)
            return head;
        ListNode * beforenode;
        ListNode * minnode, *maxnode; // find the 1st minnode
        if(head->val < x) {
            minnode = head;
        }
        else {
            beforenode = head;
            minnode = head->next;
            while(minnode != NULL && minnode->val >= x) {
                beforenode = minnode;
                minnode = minnode->next;
            }
            if(minnode == NULL) return head;
            beforenode->next = minnode->next;
            minnode->next = head; // now the head is minnode;
        }
        beforenode = minnode;
        maxnode = minnode->next;
        while(maxnode != NULL && maxnode->val < x) {
            beforenode = maxnode;
            maxnode = maxnode->next;
        }
        if(maxnode == NULL || maxnode->next == NULL) return minnode;
        // now find minnode and maxnode. the head is minnode
        head = minnode;
        ListNode * now = maxnode->next;
        while(now != NULL) {
            if(now->val >= x) {
                maxnode = now;
                now = now->next;
            }
            else {
                ListNode * tmp = beforenode->next;
                maxnode->next = now->next;
                beforenode->next = now;
                now->next = tmp;
                now = maxnode->next;
                beforenode = beforenode->next;
            }
        }
        return head;
    }
};