Linked List

Add Two Numbers

Link: https://leetcode.com/problems/add-two-numbers/

Code:

/**
 * 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 addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode p1 = l1, p2 = l2;
        ListNode preHead = new ListNode(0);
        ListNode curr = preHead;
        int carry = 0;
        int sum = 0;
        int x, y;
        while (p1 != null || p2 != null) {
            x = p1 == null ? 0 : p1.val;
            y = p2 == null ? 0 : p2.val;
            sum = x + y + carry;
            curr.next = new ListNode(sum % 10);
            carry = sum / 10;
            if (p1 != null) p1 = p1.next;
            if (p2 != null) p2 = p2.next;
            curr = curr.next;
        }
        curr.next = carry > 0 ? new ListNode(carry) : null;
        return preHead.next;
    }
}

• add one more node if the last carry is not zero;

• / operation can cast to a int type if the variable to be assigned is a int type.

• % is the remainder of the division.

Merge Two Sorted Lists

Link: https://leetcode.com/problems/merge-two-sorted-lists/

Code:

/**
 * 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 mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode p1 = list1;
        ListNode p2 = list2;
        ListNode p = new ListNode(0);
        ListNode next = p;
        while (p1 != null && p2 != null) {
            if (p1.val <= p2.val) {
                next.next = p1;
                p1 = p1.next;
            } else {
                next.next = p2;
                p2 = p2.next;
            }
            next = next.next;
        }
        next.next = p1 == null ? p2: p1;
        return p.next;
    }
}

• next = next.next; is crucial to append the linked list;

• It is not needed to append the remaining node (in list1 or list2) one by one;

• next.next = p1 == null ? p2 : p1 is another way to write if (p1 == null) { next.next = p2; } else { next.next = p1; }

Linked List Cycle II

Link: https://leetcode.com/problems/linked-list-cycle-ii/

Code:

/**
 * 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) {
        // ref: https://blog.csdn.net/y492235584/article/details/88042552
        ListNode slow = head;
        ListNode fast = head;
        while (true) {
            if (fast != null && fast.next != null) {
                fast = fast.next.next;
            } else {
                return null;
            }
            
            slow = slow.next;
            if (fast == slow) break;
        }
        
        ListNode p1 = head;
        ListNode p2 = slow;
        
        while (p1 != p2) {
            p1 = p1.next;
            p2 = p2.next;
        }
        
        return p1;
    }
}