Sort List

题目
Sort a linked list in O(n log n) time using constant space complexity.

答案

class Solution {
    public ListNode sortList(ListNode head) {
        return mergeSort(head, listLen(head));
    }
    
    private ListNode split(ListNode head, int splitSize) {
        ListNode curr = head;
        for(int i = 0; i < splitSize - 1; i++) {
            curr = curr.next;
        }
        ListNode ret = curr.next;
        curr.next = null;
        return ret;
    }
    
    private ListNode merge(ListNode head1, ListNode head2) {
        ListNode runner = new ListNode(0), start = runner, list1 = head1, list2 = head2;
        while(list1 != null || list2 != null) {
            ListNode next = null;
            if(list1 == null) {
                next = list2;
            }
            else if(list2 == null) {
                next = list1;
            }
            else {
                if(list1.val < list2.val)
                    next = list1;
                else
                    next = list2;
            }
            runner.next = next;
            runner = next;
        }
        return start.next;
    }
    
    private ListNode mergeSort(ListNode head, int len) {
        // Split head into two halves
        int splitSize = len / 2;
        ListNode list2 = split(head, splitSize);
        
        ListNode sort1 = mergeSort(head, splitSize);
        ListNode sort2 = mergeSort(list2, len - splitSize);
        
        ListNode merged = merge(sort1, sort2);
    }
    
    private int listLen(ListNode head) {
        ListNode curr = head;
        int cnt = 0;
        while(curr != null) { cnt++; curr = curr.next;}
        return cnt;
    }
}