public class LeetCode24 {
    public ListNode swapPairs(ListNode head) {
        ListNode pre = new ListNode(0);
        pre.next = head;
        ListNode temp = pre;
        while(temp.next != null && temp.next.next != null) {
            ListNode start = temp.next;
            ListNode end = temp.next.next;
            temp.next = end;
            start.next = end.next;
            end.next = start;
            temp = start;
        }
        return pre.next;
    }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
       ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode fast = dummy, slow = dummy;
        int i = 0;
        for (; i <= n  && fast != null; i++) {
            fast = fast.next;
        }
        while (fast != null) {
            fast = fast.next;
            slow = slow.next;
        }
        if (slow != null && slow.next != null) {
            slow.next = slow.next.next;
        }
        return dummy.next;
    }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        ListNode first = headA, second = headB;
        while (first != second) {
            first = (first !=null) ? first.next : headB;
            second = (second != null) ? second.next : headA;
        }
        return first;
    }
}

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        // 初始化的时候,均指向头结点
        ListNode fast = head, slow = head;
        while (true) {
            // 快指针碰到null,说明不存在环
            if (fast == null || fast.next == null) {
                return null;
            }
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) {
                break;
            }
        }
        // 重新让快指针回到头结点,快慢指针再次相遇时，其距离一定等于a + n * b
        fast = head;
        while (fast != slow) {
            fast = fast.next;
            slow = slow.next;
        }
        return fast;
    }
}