leetcode 148 Sort List
z
 2018-03-03 21:41:30  

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

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode sortList(ListNode head) {
        if(head == null || head.next == null)
            return head;
        ListNode slow = head;
        ListNode fast = head;
        ListNode mid = null;
        while(fast!= null && fast.next != null){
            mid = slow;
            slow = slow.next;
            fast = fast.next.next;
        }
        mid.next = null;
        
        ListNode p = sortList(head);
        ListNode q = sortList(slow);
        ListNode ans = merge(p,q);
        return ans;
    }
    
    public ListNode merge(ListNode p, ListNode q){
        ListNode t = new ListNode(0);
        ListNode dummy = t;
        while(p!=null && q != null){
            if(p.val < q.val){
                t.next = p;
                p = p.next;
            }
            else{
                t.next = q;
                q = q.next;
            }
            t = t.next;
        }
        if(p!=null) t.next = p;
        if(q!=null) t.next = q;
        
        return dummy.next;
    }
}
  • 利用并归排序实现的。
  • 严格意义上来说，空间复杂度并不是O(1)， 因为递归调用时，栈中同样消耗了空间。