[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) {        if (headA == NULL || headB == NULL)            return NULL;        int i = 1;        int j = 1;        ListNode* p = headA;        ListNode* q = headB;        while (p->next != NULL)        {            i++;            p = p->next;        }        while (q->next != NULL)        {            j++;            q = q->next;        }        if (p != q)            return NULL;        if (i >= j)        {            for (int k = 0; k != i-j; ++k)                headA = headA->next;        }        else        {            for (int k = 0; k != j-i; ++k)                headB = headB->next;        }        while (headA != headB)        {            headA = headA->next;            headB = headB->next;        }        return headA;    }};