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.

/**
 * 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) {
        if(headA == null || headB == null) return null;

        int a = 0;
        int b = 0;
        ListNode ha = headA;
        ListNode hb = headB;
        while(ha != null || hb != null){
            if(ha != null){
                ha = ha.next;
                a++;
            }
            if(hb != null){
                hb = hb.next;
                b++;
            }
        }

        int delta = b - a;
        ha = headA;
        hb = headB;
        if(delta > 0){
            while(delta-- > 0){
                hb = hb.next;
            }
        }else if(delta < 0){
            while(delta++ <0){
                ha = ha.next;
            }
        }
        // ping ha and hb to the right pos in the list, so that both has k distance to the end.

        while(ha != null){
            if(ha == hb) return ha;
            ha = ha.next;
            hb = hb.next;
        }

        return null;

    }
}