LeetCode Merge k Sorted Lists 解题报告

标签：leetcode解题报告   算法   java   递归   面试题   

解题报告：先不考虑任何优化，最朴素的实现方法就是遍历一遍List<ListNode>，对其中每个链表同当前链表做一遍类似于归并排序最后一步的merge操作。
算法复杂度是O(KN)

public class Solution {
    ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode head    = new ListNode(-1);
        ListNode current = head;
        while(list1!=null&&list2!=null) {
            if(list1.val<list2.val) {
                current.next = list1;
                list1   = list1.next;
            } else {
                current.next = list2;
                list2   = list2.next;
            }
            current = current.next;
        }
        if(list1!=null) {
            current.next = list1;
        } else {
            current.next = list2;
        }
        return head.next;
    }
    public ListNode mergeKLists(List<ListNode> lists) {
        if(lists==null||lists.size()==0) {
            return null;
        }
        ListNode head = lists.get(0);
        for(int i=1;i<lists.size();i++) {
            head = mergeTwoLists(head, lists.get(i));
        }
        return head;
    }
}

所以借鉴归并递归的方法，以下代码顺利AC了。时间复杂度为：O(NlogK)

public class Solution {
    ListNode merge2Lists(ListNode list1, ListNode list2) {
        ListNode head    = new ListNode(-1);
        ListNode current = head;
        while(list1!=null&&list2!=null) {
            if(list1.val<list2.val) {
                current.next = list1;
                list1   = list1.next;
            } else {
                current.next = list2;
                list2   = list2.next;
            }
            current = current.next;
        }
        if(list1!=null) {
            current.next = list1;
        } else {
            current.next = list2;
        }
        return head.next;
    }
    public ListNode mergeKLists(List<ListNode> lists) {
        if(lists==null||lists.size()==0) {
            return null;
        }
        if(lists.size()==1) {
            return lists.get(0);
        }
        int length = lists.size() ;
        int mid = (length - 1)/2 ;
        ListNode l1 = mergeKLists(lists.subList(0,mid + 1)) ;
        ListNode l2 = mergeKLists(lists.subList(mid + 1,length)) ;

        return merge2Lists(l1,l2) ;
    }
}

LeetCode Merge k Sorted Lists 解题报告

标签：leetcode解题报告   算法   java   递归   面试题   

原文地址：http://blog.csdn.net/worldwindjp/article/details/39989005