LeetCode *** 160. Intersection of Two Linked Lists

题目：

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.

分析：

该题主要是在于两个list只有相交，没有分离，所以利用这点就可以先比较AB的长度，然后从末尾开始对齐，来进行比较每个点的距离。

代码：

/**
* 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) {

if(!headA||!headB)return NULL;

int lengA=0,lengB=0;

ListNode *A=headA,*B=headB;

while(A||B){
if(A){
lengA++;
A=A->next;
}
if(B){
lengB++;
B=B->next;
}
}

A=headA,B=headB;

while(lengA>lengB){
A=A->next;
lengA--;
}

while(lengA<lengB){
B=B->next;
lengB--;
}

ListNode* res=NULL;

while(A!=B){
A=A->next;
B=B->next;
}

if(A==B)res=A;

return res;

}

};