148. Sort List

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

解法1： Merge Sort

考察了几个基本算法：
linkedlist找中点
linkedlist merge

C++

1

Java

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        // Cut into half, i.e. look for the middle point
        ListNode slow = head, fast = head.next;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        // break from the middle point
        ListNode rightHead = slow.next;
        slow.next = null;
        // Sort on each half
        ListNode left = sortList(head);
        ListNode right = sortList(rightHead);
        // Merge
        return merge(left, right);        
    }
    private ListNode merge(ListNode left, ListNode right) {
        ListNode dummy = new ListNode(0);
        ListNode tail = dummy;
        while (left != null && right != null) {
            if (left.val < right.val) {
                tail.next = left;
                left = left.next;
            } else {
                tail.next = right;
                right = right.next;
            }
            tail = tail.next;
        }
        if (left != null) {
            tail.next = left;
        }
        if (right != null) {
            tail.next = right;
        }
        return dummy.next;
    }
}