[LeetCode]Intersection of Two Linked Lists

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.

---

本题难度Easy。

双指针法

【复杂度】
时间 O(N) 空间 O(1)

【思路】
先算出两个链表各自的长度，然后从较长的链表先遍历，遍历到较长链表剩余长度和较短链表一样时，用两个指针同时遍历两个链表。这样如果链表有交点的话，两个指针已经一定会相遇。

【代码】

public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        ListNode nodea = headA, nodeb = headB;
        int lena = 0, lenb = 0;
        // 计算链表A的长度
        while(nodea != null){
            lena++;
            nodea = nodea.next;
        }
        // 计算链表B的长度
        while(nodeb != null){
            lenb++;
            nodeb = nodeb.next;
        }
        // 让较长的链表先飞一会
        for(int i = 0; i < Math.abs(lena - lenb); i++){
            if(lenb > lena) headB = headB.next;
            else            headA = headA.next;
        }
        while(headA != null && headB != null){
            if(headA == headB) return headA;
            headA = headA.next;
            headB = headB.next;
        }
        return null;
    }
}

参考

[Leetcode] Intersection of Two Linked Lists 链表交点