## Question

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.

## Solution

Result: Accepted Time: 36 ms

Here should be some explanations.

/**
* struct ListNode {
*     int val;
*     struct ListNode *next;
* };
*/
int cnt = 0;
while(a && b)
{
a = a->next;
b = b->next;
cnt ++;
}
if(a == NULL)
{
while(b)
tmp = tmp->next,
b = b->next;
b = tmp;
}
if(b == NULL)
{
while(a)
a = a->next,
tmp = tmp->next;
a = tmp;
}
while(a != b && a && b)
{
a = a->next;
b = b->next;
}
if(a && b) return a;
return NULL;
}


Complexity Analytics

• Time Complexity: $O(n)$
• Space Complexity: $O(1)$