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

For example, the following two linked lists:

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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.

O(n) time O(n) space Hashtable solution

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you know how to implement it ~

O(n) time O(1) space double iteration

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        ListNode l1 = headA, l2 = headB;
        while (l1 != l2) {
            // reset the pointer to the head of the other list when reaches the end, so both pointers traverse the same number of nodes
            l1 = l1 == null ? headB : l1.next;
            l2 = l2 == null ? headA : l2.next;
        }
        return l1;
    }
}