单链表题目*4

//获取单链表有效结点个数
public static int getLength(ListNode head){
    if(head.next == null){
        return 0;
    }
    int result = 0;
    ListNode temp = head.next;
    while (temp != null){
        result ++;
        temp = temp.next;
    }
    return result;
}

//查找单链表的倒数第k个结点
public static ListNode getLastK(int k, ListNode head){
    if(head.next == null){
        System.out.println("空链表...");
        return null;
    }
    int length = getLength(head);
    //校验 再遍历size-k个
    if(k<0 || k>length){
        return null;
    }
    ListNode temp = head.next;
    int result = 0;
    while (result == (length-k)){
        result ++;
        temp = temp.next;
    }

    return temp;
}

//反转链表
public static void reverseList(ListNode head){
    if(head.next == null || head.next.next == null){
        System.out.println("空链表...");
        return;
    }
    ListNode pre = null;
    ListNode temp = null;
    ListNode cur = head;
    while (cur != null) {
        temp = cur.next;
        cur.next = pre;
        pre = cur;
        cur = temp;
    }
}

//从尾到头打印单链表
public static void printFromTail(ListNode head){
    if(head.next == null){
        return;
    }
    Stack<ListNode> stack = new Stack<>();
    ListNode cur = head.next;
    while (cur != null){
        stack.push(cur);
        cur = cur.next;
    }
    while (stack.size() > 0) {
        System.out.println(stack.pop());
    }
}