Insertion Sort List

• Time : $O(n)$
• Space : $O(1)$

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 
ListNode* reverse(ListNode* A) {
    ListNode* dummy = new ListNode(-1);
    while(A) {
        ListNode* nxt = A->next;
        A->next = dummy->next;
        dummy->next = A;
        A = nxt;
    }
    return dummy->next;
}
ListNode* Solution::insertionSortList(ListNode* A) {
    ListNode* dummy = new ListNode(INT_MAX);
    
    while(A) {
        ListNode* runner = dummy;
        ListNode* now = A;
        A = A->next;
        now->next = NULL;
        while(runner->next and runner->next->val > now->val) {
            runner = runner->next;
        }
        now->next = runner->next;
        runner->next = now;
    }
    
    return reverse(dummy->next);
}