Day05 双指针

class Solution {
    public ListNode middleNode(ListNode head) {
        ListNode p = head;
        ListNode q = head;

        while (p != null && p.next != null) {
            p = p.next.next;
            q = q.next;
        }

        return q;
    }
}

class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode p = head;
        ListNode q = head;

        for(int i = 0; i < n; i++){
            p = p.next;
        }

        // 如果p结点此时为null，说明需要删除的是头结点，因为q指向head
        if(p == null)
            return head.next;

        while(p.next != null){
            p = p.next;
            q = q.next;
        }

        q.next = q.next.next;
        return head;
    }
}

快慢指针动画展示此。