getIntersectionNode.go

/* https://leetcode.com/problems/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:

https://assets.leetcode.com/uploads/2018/12/13/160_statement.png

begin to intersect at node c1.

Example 1:

https://assets.leetcode.com/uploads/2018/12/13/160_example_1.png

    Input: intersectVal = 8, listA = [4,1,8,4,5], listB = [5,0,1,8,4,5], 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 [5,0,1,8,4,5].
		There are 2 nodes before the intersected node in A; There are 3 nodes before the intersected node in B.

Example 2:

https://assets.leetcode.com/uploads/2018/12/13/160_example_2.png

    Input: intersectVal = 2, listA = [0,9,1,2,4], 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 [0,9,1,2,4].
		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:

https://assets.leetcode.com/uploads/2018/12/13/160_example_3.png

    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.
    Your code should preferably run in O(n) time and use only O(1) memory.
*/

package lll

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */

func getIntersectionNode(headA, headB *ListNode) *ListNode {
	// len(headA + headB) == len(headB + headA)
	a, b := headA, headB
	for a != b {
		if a == nil {
			a = headB
		} else {
			a = a.Next
		}
		if b == nil {
			b = headA
		} else {
			b = b.Next
		}
	}
	return a
}

/*
func getIntersectionNode(headA, headB *ListNode) *ListNode {
	countA := 0
	for a := headA; a != nil; a = a.Next {
		countA++
	}
	countB := 0
	for b := headB; b != nil; b = b.Next {
		countB++
	}

	long, short := headA, headB
	if countA < countB {
		long, short = short, long
		countA, countB = countB, countA
	}

	for diff := countA - countB; diff > 0; diff-- {
		long = long.Next
	}

	// now len(long) == len(short)
	for long != nil && long != short {
		long, short = long.Next, short.Next
	}
	return long
}
*/