LeetCode 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.

如果没有要求此题很简单。用hashset就可以。

但是题目要求 in O(n) time and use only O(1) memory。

可以先遍历两个链表，把他们的长度变为一样长。然后逐个比较。

/**
 * 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 a=0, b = 0;
        ListNode tempA = headA;
        ListNode tempB = headB;
        while (tempA != null) {
            ++a;
            tempA = tempA.next;
        }
        while (tempB != null) {
            ++b;
            tempB = tempB.next;
        }
        tempA = headA;
        tempB = headB;
        while (a > b) {
            --a;
            tempA = tempA.next;
        }
        while (b > a) {
            --b;
            tempB = tempB.next;
        }
        while (tempA != null) {
            if (tempA == tempB) {
                return tempA;
            } else {
                tempA = tempA.next;
                tempB = tempB.next;
            }
        }
        return null;
    }
}