12.3 Other Linked List Techniques

160. Intersection of Two Linked Lists

Problem Description

Given two linked lists, determine if they intersect at a point and find the intersecting node.

Input and Output Example

Input consists of two linked lists, output is a node. If there is no intersection, return nullptr.

Input:
A:        a1 -> a2
                |
                v
                c1 -> c2 -> c3
                ^
                |
B: b1 -> b2 -> b3
Output: c1

Solution Explanation

Assume the distance from the head of linked list A to the intersection is a, the distance from the head of linked list B to the intersection is b, and the distance from the intersection to the end of the lists is c.

We use two pointers starting at the heads of the two linked lists and move them forward at the same speed. When a pointer reaches the end of a list, it continues from the head of the other list. With this approach, both pointers will meet at the intersection node after a + b + c steps.

C++

ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
    ListNode *l1 = headA, *l2 = headB;
    while (l1 != l2) {
        l1 = l1 != nullptr ? l1->next : headB;
        l2 = l2 != nullptr ? l2->next : headA;
    }
    return l1;
}

Python

def getIntersectionNode(
    headA: ListNode, headB: ListNode
) -> Optional[ListNode]:
    l1 = headA
    l2 = headB    
    while l1 != l2:
        l1 = l1.next if l1 is not None else headB
        l2 = l2.next if l2 is not None else headA    
    return l1

234. Palindrome Linked List

Problem Description

Determine if a linked list is a palindrome using $O(1)$ space complexity.

Input and Output Example

Input: A linked list.
Output: A boolean indicating whether the linked list is a palindrome.

Input: 1->2->3->2->1
Output: true

Solution Explanation

First, use a slow and fast pointer to find the middle of the linked list. Then split the list into two halves. Reverse the second half and compare the two halves for equality.

C++

bool isPalindrome(ListNode* head) {
    if (head == nullptr || head->next == nullptr) {
        return true;
    }
    ListNode *slow = head, *fast = head;
    while (fast->next && fast->next->next) {
        slow = slow->next;
        fast = fast->next->next;
    }
    slow->next = reverseList(slow->next);  // Refer to problem 206.
    slow = slow->next;
    while (slow != nullptr) {
        if (head->val != slow->val) {
            return false;
        }
        head = head->next;
        slow = slow->next;
    }
    return true;
}

Python

def isPalindrome(head: Optional[ListNode]) -> bool:
    if head is None or head.next is None:
        return True

    slow, fast = head, head

    while fast.next is not None and fast.next.next is not None:
        slow = slow.next
        fast = fast.next.next

    slow.next = reverseList(slow.next)  # Refer to problem 206.
    slow = slow.next

    while slow is not None:
        if head.val != slow.val:
            return False
        head = head.next
        slow = slow.next

    return True