力扣labuladong——一刷day02

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前言

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一、力扣876. 链表的中间结点

/**
 * 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 middleNode(ListNode head) {
        ListNode res = new ListNode(-1,head);
        ListNode p = head;
        int len = 0;
        while(p != null){
            p = p.next;
            len ++;
        }
        int i = len / 2 + 1;
        p = head;
        for(int j = 1; j < i; j ++){
            p = p.next;
        }
        return p;
    }
}

快慢指针遍历一次

/**
 * 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 middleNode(ListNode head) {
        ListNode dump = new ListNode(-1,head);
        ListNode p1 = dump, p2 = dump;
        for(; p2 != null;){
            p1 = p1.next;
            p2 = p2.next;
            if(p2 == null){
                return p1;
            }
            p2 = p2.next;
        }
        return p1;
    }
}

二、力扣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 {
    public ListNode detectCycle(ListNode head) {
        ListNode p1 = head, p2 = head;
        while(p1 != null && p2 != null && p2.next != null){
            p1 = p1.next;
            p2 = p2.next.next;
            if(p1 == p2){
                p1 = head;
                while(p1 != p2){
                    p1 = p1.next;
                    p2 = p2.next;
                }
                return p1;
            }
        }
        return null;
    }
}

三、力扣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) {
        ListNode p1 = headA, p2 = headB;
        int len1 = 0, len2 = 0;
        while(p1 != null){
            len1 ++;
            p1 = p1.next;
        }
        while(p2 != null){
            len2 ++;
            p2 = p2.next;
        }
        ListNode fast, slow;
        int edge = 0;
        if(len1 > len2){
            edge = len1 - len2;
            fast = headA;
            slow = headB;
        }else{
            edge = len2 - len1;
            fast = headB;
            slow = headA;
        }
        while(edge > 0){
            edge --;
            fast = fast.next;
        }

        while(fast != null && slow != null){
            if(fast == slow){
                return fast;
            }
            fast = fast.next;
            slow = slow.next;
        }
        return null;
    }
}

双指针

/**
 * 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 p1 = headA, p2 = headB;
        while(p1 != p2){
            if(p1 == null) p1 = headB;
            else p1 = p1.next;
            
            if(p2 == null) p2 = headA;
            else p2 = p2.next;
        }
        return p1;
    }
}

四、力扣141. 环形链表

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode p1 = head, p2 = head;
        while(p1 != null && p2 != null  && p2.next != null){
            p1 = p1.next;
            p2 = p2.next.next;
            if(p1 == p2){
                return true;
            }
        }
        return false;
    }
}