[LeetCode] Insertion Sort List

Well, life gets difficult pretty soon whenever the same operation on array is transferred to linked list.

First, a quick recap of insertion sort:

Start from the second element (simply a[1] in array and the annoying head -> next -> val in linked list), each time when we see a node with val smaller than its previous node, we scan from the head and find the position that the current node should be inserted. Since a node may be inserted before head, we create a new_head that points to head. The insertion operation, however, is a little easier for linked list.

Now comes the code:

class Solution { 
public:
    ListNode* insertionSortList(ListNode* head) {
        ListNode* new_head = new ListNode(0);
        new_head -> next = head;
        ListNode* pre = new_head;
        ListNode* cur = head;
        while (cur) {
            if (cur -> next && cur -> next -> val < cur -> val) {
                while (pre -> next && pre -> next -> val < cur -> next -> val)
                    pre = pre -> next;
                /* Insert cur -> next after pre.*/
                ListNode* temp = pre -> next;
                pre -> next = cur -> next;
                cur -> next = cur -> next -> next;
                pre -> next -> next = temp;
                /* Move pre back to new_head. */
                pre = new_head;
            }
            else cur = cur -> next;
        }
        ListNode* res = new_head -> next;
        delete new_head;
        return res;
    }
};