Intersection of Two Linked Lists

Description

Write a program to find the node at which the intersection of two singly linked lists begins.

For example, the following two linked lists:

begin to intersect at node c1.

 

Example 1:

Input: intersectVal = 8, listA = [4,1,8,4,5], listB = [5,6,1,8,4,5], skipA = 2, skipB = 3
Output: Reference of the node with value = 8
Input Explanation: The intersected node's value is 8 (note that this must not be 0 if the two lists intersect). From the head of A, it reads as [4,1,8,4,5]. From the head of B, it reads as [5,6,1,8,4,5]. There are 2 nodes before the intersected node in A; There are 3 nodes before the intersected node in B.

 

Example 2:

Input: intersectVal = 2, listA = [1,9,1,2,4], listB = [3,2,4], skipA = 3, skipB = 1
Output: Reference of the node with value = 2
Input Explanation: The intersected node's value is 2 (note that this must not be 0 if the two lists intersect). From the head of A, it reads as [1,9,1,2,4]. From the head of B, it reads as [3,2,4]. There are 3 nodes before the intersected node in A; There are 1 node before the intersected node in B.

 

Example 3:

Input: intersectVal = 0, listA = [2,6,4], listB = [1,5], skipA = 3, skipB = 2
Output: null
Input Explanation: From the head of A, it reads as [2,6,4]. From the head of B, it reads as [1,5]. Since the two lists do not intersect, intersectVal must be 0, while skipA and skipB can be arbitrary values.
Explanation: The two lists do not intersect, so return null.

 

Notes:

• If the two linked lists have no intersection at all, return null.
• The linked lists must retain their original structure after the function returns.
• You may assume there are no cycles anywhere in the entire linked structure.
• Each value on each linked list is in the range [1, 10^9].
• Your code should preferably run in O(n) time and use only O(1) memory.

Solution(javascript)

// O(n) time O(n) space
// const getIntersectionNode = (headA, headB) => {
//   const map = new Map()
//   let current = headA
//   while (current) {
//     map.set(current, true)
//     current = current.next
//   }
//   current = headB
//   while (current) {
//     if (map.get(current)) {
//       return current
//     }
//     current = current.next
//   }
//   return null
// }


const getIntersectionNode = (headA, headB) => {
  const getLength = (head) => {
    let length = 0
    let current = head
    while (current) {
      length += 1
      current = current.next
    }
    return length
  }
  let lengthA = getLength(headA)
  let lengthB = getLength(headB)
  while (lengthA !== lengthB) {
    if (lengthA > lengthB) {
      lengthA -= 1
      headA = headA.next
    } else {
      lengthB -= 1
      headB = headB.next
    }
  }
  while (headA !== headB) {
    headA = headA.next
    headB = headB.next
  }
  return headA
}