[LeetCode] Merge k Sorted Lists

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

递归归并

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeKLists(List<ListNode> lists) {
        if (lists == null || lists.size() < 1) {
            return null;
        }
        
        if (lists.size() == 1)  {
            return lists.get(0);
        }
        
        List<ListNode> llists = new ArrayList<ListNode>();
        for (int i = 0; i < lists.size() / 2; i++) {
            llists.add(lists.get(i));
        }
        
        List<ListNode> rlists = new ArrayList<ListNode>();
        for (int i = lists.size() / 2; i < lists.size(); i++) {
            rlists.add(lists.get(i));
        }
        
        ListNode left = mergeKLists(llists);
        ListNode right = mergeKLists(rlists);
        return merge(left, right);
    }
    
    private ListNode merge(ListNode left, ListNode right) {
        if (left == null) {
            return right;
        }
        
        if (right == null) {
            return left;
        }
        
        ListNode p1 = left;
        ListNode p2 = right;
        ListNode head = new ListNode(0);
        ListNode cur = head;
        while (p1 != null && p2 != null) {
            if (p1.val < p2.val) {
                cur.next = p1;
                p1 = p1.next;
            } else {
                cur.next = p2;
                p2 = p2.next;
            }
            cur = cur.next;
        }
        
        if (p1 != null) {
            p2 = p1;
        }
        
        while (p2 != null) {
            cur.next = p2;
            cur = cur.next;
            p2 = p2.next;
        }
        
        return head.next;
    }
}