Leetcode||23. Merge k Sorted Lists

23. Merge k Sorted Lists

• Total Accepted: 116272
• Total Submissions: 455841
• Difficulty: Hard
• Contributors: Admin

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

这题和21题类似，一开始我逐个归并，直接超时，使用二分法归并就AC，还是二分大法好啊

代码如下：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
		int len = lists.length;
		if (len <= 0) {
			return null;
		}
		if (len == 1) {
			return lists[0];
		}
		while (len>1) {
			int mid=(len+1)/2;
		    for(int i=0;i<len/2;i++)
		    {
		    	lists[i]=mergeTwoLists(lists[i],lists[i+mid]);
		    }
		    len=mid;
		}
		return lists[0];
    }
	public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode result = new ListNode(0);
        ListNode lastNode = result;
        while (l1 != null && l2 != null) {
			if (l1.val < l2.val) {
				lastNode.next = l1;
				l1 = l1.next;
			} else {
				lastNode.next = l2;
				l2 = l2.next;
			}
			lastNode = lastNode.next;
		}
        if (l1 != null) {
			lastNode.next = l1;
		} else {
			lastNode.next = l2;
		}
        return result.next;
    }
}