[算法] 將單鏈表的每K個節點之間逆序

題目

給定一個單鏈表的頭結點，實現一個調整單鏈表的函數，使得每K個節點之間逆序，若是最後不夠K個節點一組，則不調整最後幾個節點。

解答

使用棧結構

import java.util.Stack;
 public class Test{
 static class Node{
    public int val;
    public Node next;
    public Node(int val){
        this.val=val;
    }
}
public static void main(String[] args) {
    Node head=new Node(1);
    head.next=new Node(2);
    head.next.next=new Node(3);
    head.next.next.next=new Node(4);
    head.next.next.next.next=new Node(5);
    head.next.next.next.next.next=new Node(6);
    Node node=reverseKNodes(head,2);
    while(node!=null){
        System.out.print(node.val+" ");
        node=node.next;
    }
}

public static Node reverseKNodes(Node head, int K){
    if (K<2) {
        return head;
    }
    Stack<Node> stack=new Stack<>();
    Node newHead=head;
    Node cur=head;
    Node pre=null;
    Node next=null;
    while(cur!=null){
        stack.push(cur);
        next=cur.next;
        if (stack.size()==K) {
            pre=resign(stack,pre,next);
            newHead=newHead==head?cur:newHead;
        }
        cur=next;
    }
    return newHead;
}
public static Node resign(Stack<Node> stack,Node left,Node right){
        Node cur=stack.pop();
        if (left!=null) {
            left.next=cur;
        }
        Node next=null;
        while(!stack.isEmpty()){
            next=stack.pop();
            cur.next=next;
            cur=next;
        }
        cur.next=right;
        return cur;
}
 }

輸出：2 1 4 3 6 5

不使用棧結構

import java.util.Stack;
public class Test{
 static class Node{
    public int val;
    public Node next;
    public Node(int val){
        this.val=val;
    }
}
public static void main(String[] args) {
    Node head=new Node(1);
    head.next=new Node(2);
    head.next.next=new Node(3);
    head.next.next.next=new Node(4);
    head.next.next.next.next=new Node(5);
    head.next.next.next.next.next=new Node(6);
    Node node=reverseKNodes(head,2);
    while(node!=null){
        System.out.print(node.val+" ");
        node=node.next;
    }
}

public static Node reverseKNodes(Node head, int K){
    if (K<2) {
        return head;
    }
    Node cur=head;
    Node pre=null;
    Node next=null;
    Node start=null;
    int count=1;
    while(cur!=null){
        next=cur.next;
        if (count==K) {
            start=pre==null?head:pre.next;
            head=pre==null?cur:head;
            resign(pre,start,cur,next);
            pre=start;
            count=0;
        }
        count++;
        cur=next;
    }
    return head;
}
public static void resign(Node left,Node start,Node end, Node right){
    Node pre=start;
    Node cur=start.next;
    Node next=null;
    while(cur!=right){
        next=cur.next;
        cur.next=pre;
        pre=cur;
        cur=next;
    }
    if (left!=null) {
        left.next=end;
    }
    start.next=right;
}

}