160. Intersection of Two Linked Lists

I count the number of the two lists first. And then I compute the offset and move the head pointer of the longer list to the offset. And from there compare the two lists and check the intersection.

1:  /**  
2:   * Definition for singly-linked list.  
3:   * struct ListNode {  
4:   *   int val;  
5:   *   ListNode *next;  
6:   *   ListNode(int x) : val(x), next(NULL) {}  
7:   * };  
8:   */  
9:  class Solution {  
10:  public:  
11:    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {  
12:      int a = countList(headA);  
13:      int b = countList(headB);  
14:      int diff = 0;  
15:      if (a > b) {  
16:        diff = a - b;  
17:        while (diff) { headA = headA->next; diff--; }  
18:      } else {  
19:        diff = b - a;  
20:        while (diff) { headB = headB->next; diff--; }  
21:      }  
22:      while (headA != headB) {  
23:        headA = headA->next;  
24:        headB = headB->next;  
25:      }  
26:      return headA;  
27:    }  
28:    int countList(ListNode *head) {  
29:      int count = 0;  
30:      while (head) {  
31:        head = head->next;  
32:        count++;  
33:      }  
34:      return count;  
35:    }  
36:  };