代码随想录二刷day04

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前言

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一、力扣24. 两两交换链表中的节点

迭代

/**
 * 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) {
        ListNode node = new ListNode(-1, head);
        ListNode prev = node, cur = head;
        int temp;
        while(cur != null && cur.next != null){
            temp = cur.val;
            cur.val = cur.next.val;
            cur.next.val = temp;
            prev = cur.next;
            cur = prev.next;
        }
        return node.next;
    }
}

递归

/**
 * 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) {
        if(head == null || head.next == null){
            return head;
        }
        int temp;
        ListNode cur = head;
        temp = cur.val;
        cur.val = cur.next.val;
        cur.next.val = temp;
        ListNode node = swapPairs(cur.next.next);
        if(node == null){
            return cur;
        }
        return cur;
    }
}

二、力扣19. 删除链表的倒数第 N 个结点

直观法

/**
 * 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) {
        if(head == null){
            return head;
        }
        ListNode node = new ListNode(-1, head);
        ListNode pre = node, p = head;
        int i = 0, size = 0;
        while(p != null){
            size ++;
            p = p.next;
        }
        if(n > size){
            return head;
        }
        p = head;
        while(i < size-n){
            i ++;
            pre = p;
            p = p.next;
        }
        pre.next = p.next;
        return node.next;
    }
}

双指针法

/**
 * 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 node = new ListNode(-1, head);
        ListNode fast = node, slow = node;
        int i = 1;
        while(i <= n+1 && fast != null){
            i ++;
            fast = fast.next;
        }
        if(fast == null && i < n+1){
            return node.next;
        }
        while(fast != null){
            fast = fast.next;
            slow = slow.next;
        }
        slow.next = slow.next.next;
        return node.next;
    }
}

三、力扣面试题 02.07. 链表相交

/**
 * 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 longL = new ListNode(-1, headA);
        ListNode shortL = new ListNode(-1, headB);
        ListNode pa = headA, pb = headB;
        int lenA = 0, lenB = 0, diff = 0;
        while(pa != null){
            lenA ++;
            pa = pa.next;
        }
        while(pb != null){
            lenB ++;
            pb = pb.next;
        }
        if(lenA > lenB){
            diff = lenA - lenB;
            longL.next = headA;
            shortL.next = headB;
        }else{
            diff = lenB - lenA;
            longL.next = headB;
            shortL.next = headA;
        }
        pa = longL.next; pb = shortL.next;
        for(int i = 0; i < diff; i++){
            pa = pa.next;
        }
        while(pa != null && pb != null){
            if(pa == pb){
                return pa;
            }
            pa = pa.next;
            pb = pb.next;
        }
        return null;
    }
}

四、力扣142. 环形链表 II

因为快指针的速度是慢指针的两倍， 所以，在进入环形之后， 在快慢指针相遇之前， 慢指针必然走不满一圈，设入环之前的节点数为x, 入环后到相遇节点之前为y个， 相遇节点到入环处为z个， 相遇时， 快指针走过的节点数是慢指针的2倍， 故有，2(x + y) = x + n(z + y) + y,， 化简后为， x = n(z +y)-y， 右边提取一个 y 得到，x = (n-1)(z+y) + z， 此时的慢指针，一定会先走完z， 然后就是n-1圈， 就一直在进入点等待x

/**
 * 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) {
        ListNode fastNode = head, slowNode = head;
        while(fastNode != null && fastNode.next != null){
            fastNode = fastNode.next.next;
            slowNode = slowNode.next;
            if(fastNode == slowNode){
                ListNode index1 = head;
                ListNode index2 = slowNode;
                while(index1 != index2){
                    index1 = index1.next;
                    index2 = index2.next;
                }
                return index1;
            }
        }
        return null;
    }
}