代码随想录算法训练营第四天| 24. 两两交换链表中的节点 19.删除链表的倒数第N个节点 160. 相交链表 142. 环形链表 II

24. 两两交换链表中的节点

24. 两两交换链表中的节点

/**
 * 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 swapPairs(ListNode head) {
        //使用双指针+虚拟头结点的方法亮亮交换链表中的节点
        //注意：画图确定交换的顺序
        ListNode dummy = new ListNode();
        dummy.next = head;
        ListNode cur = dummy;  
        while(cur.next != null && cur.next.next !=null){
            ListNode tmp = cur.next;
            ListNode tmp1 = cur.next.next.next;
            //开始交换
            cur.next = cur.next.next;
            cur.next.next = tmp;
            tmp.next.next.next = tmp1;
            //准备开始下一组交换
            cur = cur.next.next;

        }
        return dummy.next;
    }
}

19.删除链表的倒数第N个节点

19.删除链表的倒数第N个节点

/**
 * 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) {
        //链表中双指针的经典应用，快慢指针，先让快指针fast 移动n步，再将slow和fast同时移动，直到fast指向Null
        //首先创建虚拟头节点
        ListNode dummy = new ListNode();
        dummy.next = head;
        ListNode slow = dummy;
        ListNode fast = dummy;
        for(int i = 0;i < n; i++){
            fast = fast.next;
        }
          //此处fast节点移动n+1次，可以保证当fast移动到最后null时候，slow刚好指向待删除节点的前一个节点，方便了删除的过程
        fast = fast.next;
        while(fast != null){
            slow = slow.next;
            fast = fast.next;
        }
     
        //此时slow指向待删除节点的前一个节点
        slow.next = slow.next.next;
        return dummy.next;

    }
}

160. 相交链表

160. 相交链表

/**
 * 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) {
     // p1 指向 A 链表头结点，p2 指向 B 链表头结点
    ListNode p1 = headA, p2 = headB;
    while (p1 != p2) {
        // p1 走一步，如果走到 A 链表末尾，转到 B 链表
        if (p1 == null) p1 = headB;
        else            p1 = p1.next;
        // p2 走一步，如果走到 B 链表末尾，转到 A 链表
        if (p2 == null) p2 = headA;
        else            p2 = p2.next;
    }
    return p1;
    }
}

142. 环形链表 II

142. 环形链表 II

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    ListNode detectCycle(ListNode head) {
    ListNode fast, slow;
    fast = slow = head;
    while (fast != null && fast.next != null) {
        fast = fast.next.next;
        slow = slow.next;
        if (fast == slow) break;
    }
    // 上面的代码类似 hasCycle 函数
    if (fast == null || fast.next == null) {
        // fast 遇到空指针说明没有环
        return null;
    }

    // 重新指向头结点
    slow = head;
    // 快慢指针同步前进，相交点就是环起点
    while (slow != fast) {
        fast = fast.next;
        slow = slow.next;
    }
    return slow;
}

}