Leetcode 21-25

这几道题都是典型的listnode节点类型问题

21

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1==null)return l2;
        if(l2==null)return l1;
        ListNode a=l1;
        ListNode b=l2;
        ListNode head=null;
        if(l1.val<l2.val){
            head=l1;
            a=a.next;
        }else{
            head=l2;
            b=b.next;
        }
        ListNode p=head;
        while(a!=null&&b!=null){
            if(a.val>b.val){
                p.next=b;
                p=p.next;
                b=b.next;
            }else{
                p.next=a;
                p=p.next;
                a=a.next;
            }
        }
        if(a==null&&b==null){
            return head;
        }else if(a==null&&b!=null){
            p.next=b;
        }else{
            p.next=a;
        }
        return head; 
        
        
    }
}

23

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists == null || lists.length == 0) return null;
        if (lists.length == 1) return lists[0];
        PriorityQueue<ListNode> queue = new PriorityQueue<>(new Comparator<ListNode>() {
            public int compare(ListNode l1, ListNode l2) {
                return l1.val - l2.val;
            }
        });
        for (int i = 0; i < lists.length; i++) {
            if (lists[i] != null) queue.add(lists[i]);
        }
        ListNode dummy = new ListNode(-1), cur = dummy;
        while (!queue.isEmpty()) {
            ListNode temp = queue.poll();
            cur.next = temp;
            cur = temp;
            if (temp.next != null) queue.add(temp.next);
        }
        return dummy.next;        
    }
}

24

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head==null)
            return null;
        
        //表头节点，默认链表中的数据都为正数
        ListNode preHead = new ListNode(-1);
        preHead.next=head;
       
        ListNode left=preHead;
        ListNode mid=head;
        if(head.next==null)             
            return head;
        ListNode right=head.next;
        
        while(mid!=null && mid.next!=null){
            
            mid.next=right.next;    
            right.next=mid;
            left.next=right;
           
            left=mid;          
            mid=left.next;
            if(mid!=null)         
                right=mid.next;
          
            
        }
        
        return preHead.next;
    }
}

25

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode root = new ListNode(-1);
		root.next = head;
		ListNode res = root;
		ListNode temp = head;
		int i = 0;
		while(temp != null){
			i++;
			temp = temp.next;
		}
		while(i >= k){
			for(int j = 0 ; j < k-1; j++){
				ListNode node = root.next;
				root.next = head.next;
				head.next = root.next.next;
				root.next.next = node;
			}
			root = head;
			head = head.next;
			i-=k;
		}
		return res.next;
    }
}

节点问题真的是让我头疼很久，指针，节点移动不画图完全搞不懂

不过多做几次，还是挺容易看懂的。