LeetCode 160. 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.

/** * 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) {        int m = 0, n = 0;        ListNode *listA = headA;        ListNode *listB = headB;        while (listA != NULL) {            listA = listA->next;            ++m;        }        while (listB != NULL) {            listB = listB->next;            ++n;        }        listA = headA;        listB = headB;        while (m > n) {            --m;            listA = listA->next;        }        while (n > m) {            --n;            listB = listB->next;        }        while (n > 0) {            if (listA == listB) {                return listA;            }            listA = listA->next;            listB = listB->next;            --n;        }        return NULL;    }};