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.

题意：找两个链的第一个公共节点

思路：首先得到两个链的长度，然后差就相当于是相差的步数了，较长的移动步数的话，那么两个链再同时走第一个节点就是公共节点了；还有哈希和暴力两种方法

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
 	ListNode *pa=headA, *pb=headB;
			int lengthA = 0, lengthB = 0;
			while (pa) {
				pa=pa->next;
				lengthA++;
			}
			while (pb) {
				pb=pb->next;
				lengthB++;
			}
			if (lengthA <= lengthB) {
				int n = lengthB - lengthA;
				pa = headA;
				pb = headB;
				while(n) {
					pb = pb->next;
					n--;
				}
			} else{
				int n = lengthA - lengthB;
				pa = headA;
				pb = headB;
				while(n) {
					pa = pa->next;
					n--;
				}
			}
			while (pa != pb) {
				pa = pa->next;
				pb = pb->next;
			}
			return pa;
    }
};

LeetCode Intersection of Two Linked Lists