160. Intersection of Two Linked Lists

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

For example, the following two linked lists:

A:          a1 → a2
                   ↘
                     c1 → c2 → c3
                   ↗            
B:     b1 → b2 → b3

begin to intersect at node c1.

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.

//Time: O(n), Space: O(1)    
public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        if (headA == null || headB == null) {
            return null;
        }
        
        int lenA = getLen(headA);
        int lenB = getLen(headB);
        
        if (lenA > lenB) {
            headA = forward(headA, lenA - lenB);
        } else {
            headB = forward(headB, lenB - lenA);
        }
        
        
        while (headA != null && headB != null) {
            if (headA == headB) {
                return headA;
            }
            
            headA = headA.next;
            headB = headB.next;
        }
        
        return null;
    }
    
    private int getLen(ListNode head) {
        int count = 1;
        
        while (head != null) {
            head = head.next;
            count++;
        }
        
        return count;
    }
    
    private ListNode forward(ListNode head, int step) {
        while (step > 0) {
            head = head.next;
            step--;
        }
        
        return head;
    }