[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.

老题目了，复习一下。

/**
 * 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, *pB;
        int lenA, lenB;
        pA = headA; 
        lenA = 0;
        while (pA != NULL) {
            ++lenA;
            pA = pA->next;
        }
        pB = headB; 
        lenB = 0;
        while (pB != NULL) {
            ++lenB;
            pB = pB->next;
        }
        int idx = lenA > lenB ? (lenA - lenB) : (lenB - lenA);
        pA = headA; pB = headB;
        if (lenA > lenB) {
            while (idx--) {
                pA = pA->next;
            }
        } else {
             while (idx--) {
                pB = pB->next;
            }
        }
        while (pA != pB) {
            pA = pA->next;
            pB = pB->next;
        }
        return pA;
    }
};