「leetCode」：Sort List

語言：JAVA

思路：要是o(nlogn)時間內排序一個鏈表，並且要求常量空間複雜度。那麼使用歸併排序，並且將原表分成兩個表，使用原表的空間進行。

代碼：

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode sortList(ListNode head) {
        if (head==null || head.next==null) {
        	return head;
        }
        
        ListNode rightStart = head;
        ListNode curorIndex = head.next.next;
        
        while (curorIndex!=null &&curorIndex.next!=null) {
        	rightStart = rightStart.next;
        	curorIndex = curorIndex.next.next;
        }
        
        ListNode right = sortList(rightStart.next);
        rightStart.next = null;
        return merge_sort(sortList(head), right);
    }
	
    public ListNode merge_sort(ListNode left, ListNode right) {
    	ListNode head = new ListNode(Integer.MIN_VALUE);
    	ListNode tail = head;
    	
    	while (left!=null && right!=null) {
    		if (left.val > right.val) {
    			tail.next = right;
    			right = right.next;
    		} else {
    			tail.next = left;
    			left = left.next;
    		}
    		tail = tail.next;
    	}
    	
    	if (left != null)
    		tail.next = left;
    	if (right != null)
    		tail.next = right;
    	
    	return head.next;
    }
}