160.Intersection of Two Linked Lists

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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:

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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.

Answer

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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) {
        int lenA = getLength(headA);
        int lenB = getLength(headB);
        while(lenA<lenB){
            headB = headB.next;
            lenB--;
        }
        while(lenA>lenB){
            headA = headA.next;
            lenA--;
        }
        while(headA!=headB){
            headA = headA.next;
            headB = headB.next;
        }
        return headA;
    }
    public int getLength(ListNode node){
        int length = 0;
        while(node!=null){
            length++;
            node = node.next;
        }
        return length;
    }
}