Day 92, ##, Intersection of Two Linked Lists

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.

Credits:
Special thanks to @stellari for adding this problem and creating all test cases.
------------------------------------------------------------
Solution #1, attach A's end to A's start. solve it as finding the start of cycle in a linked list

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        if (headA == NULL) return NULL;
        ListNode *end =headA;
        while (end->next != NULL) {
            end = end->next;
        }
        end->next = headA;
        
        ListNode *fast = headB, *slow = headB;
        while (fast != NULL) {
            fast = fast->next;
            slow = slow->next;
            if (fast != NULL) {
                fast = fast->next;
            }else {
                end->next = NULL;
                return NULL;
            }
            
            if (slow == fast) break;
        }
        
        if (fast == NULL) {
            end->next = NULL;
            return NULL;
        }
        
        fast = headB;
        while (fast != slow) {
            slow = slow->next;
            fast = fast->next;
        }
        end->next = NULL;
        
        return fast;
    }
};

Solution #2
explanation https://oj.leetcode.com/problems/intersection-of-two-linked-lists/solution/

class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        ListNode *endA = NULL, *endB = NULL;
        ListNode *itrA = headA, *itrB = headB;
        
        while (itrA != NULL && itrB != NULL) {
            if (endA != NULL && endB != NULL && endA != endB) return NULL;
            if (itrA == itrB) return itrA;
            
            if (itrA->next == NULL) {
                if (endA == NULL) endA = itrA;
                itrA = headB;
            }else {
                itrA = itrA->next;
            }
            
            if (itrB->next == NULL) {
                if (endB == NULL) endB = itrB;
                itrB = headA;
            }else {
                itrB = itrB->next;
            }
        }
        
        return NULL;
    }
};