LeetCode--23. Merge k Sorted Lists

合并k个已经排序的列表

列表的数据结构：

class ListNode {
    int val;
    ListNode next;

    ListNode(int x) {
        val = x;
    }
}

方法1：将后面链表里面的元素插入到前面的链表中，时间复杂度O(K*N)

public static ListNode mergeKLists(ListNode[] lists) {
    int len = lists.length;
    ListNode[] startarr = new ListNode[len];
    for(int i = 0 ; i < len ; i++){
        startarr[i] = lists[i];

    }
    
    ListNode head = new ListNode(Integer.MIN_VALUE);
    head.next = null;
    ListNode p = head; //当前指针
    ListNode pre = head; // 前一个指针
    ListNode temp = null; 
    
    for(int i = 0 ; i < len ; i++){
        p=head;
        pre = head;
        while(startarr[i]!=null&&p!=null){
            if(startarr[i].val>p.val){
                pre = p;
                p = p.next;
            }else{ //将元素插入到前面的链表中
                temp = startarr[i].next;
                startarr[i].next= p;
                pre.next=startarr[i];
                startarr[i]=temp;
                p=pre.next;                
            }
        }
        
        if(p==null){ //如果p已经到头了，将当前链表剩下的元素直接连在p后面
            pre.next = startarr[i];
        }
    }                
    return head.next;
}

测试：

public static void main(String[] args) {
    ListNode n11 = new ListNode(1);
    ListNode n12 = new ListNode(3);
    ListNode n13 = new ListNode(5);
    ListNode n14 = new ListNode(7);
    ListNode n15 = new ListNode(7);
    n11.next = n12;
    n12.next = n13;
    n13.next = n14;
    n14.next = n15;
    n15.next = null;
    
    
    ListNode n21 = new ListNode(2);
    ListNode n22 = new ListNode(4);
    ListNode n23 = new ListNode(6);
    ListNode n24 = new ListNode(6);
    ListNode n25 = new ListNode(10);
    n21.next = n22;
    n22.next = n23;
    n23.next = n24;
    n24.next = n25;
    n25.next = null;
    
    ListNode[] lists =new ListNode[2];
    lists[0]=n11;
    lists[1]=n21;
    ListNode lmerge = mergeKLists(lists);

    while(lmerge!=null){
        System.out.println(lmerge.val);
        lmerge=lmerge.next;
    }
}

 

解法2：采用归并的方法

http://blog.csdn.net/worldwindjp/article/details/39989005
http://www.tuicool.com/articles/ZnuEVfJ

public static ListNode mergeKLists(ListNode[] lists) {
    if(lists==null || lists.length==0)
        return null;
    return helper(lists,0,lists.length-1);
}

private static ListNode helper(ListNode[] lists, int l, int r){
    if(l<r){
        int m = (l+r)/2;
        return mergeTwoLists(helper(lists,l,m),helper(lists,m+1,r));
    }
    return lists[l];
}

public static ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    ListNode start1 = l1 ;
    ListNode start2 = l2 ;
    ListNode head = new ListNode(0);
    head.next = null;
    ListNode end = head;
    
    //尾插法
    while(start1!=null&&start2!=null){
        if(start1.val<start2.val){
            end.next = start1;
            end = end.next;
            start1 = start1.next;
        }else{
            end.next = start2;
            end = end.next;
            start2 = start2.next;
        }
        
    }
    if(start1!=null){
        end.next = start1;            
    }
    if(start2!=null){
        end.next = start2;
    }
    return head.next;
}