[Linked List]Intersection of Two Linked Lists

Total Accepted: 53721 Total Submissions: 180705 Difficulty: Easy

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:
    int getListLength(ListNode* head){
        int len = 0;
        while(head){
            len++;
            head=head->next;
        }
        return len;
    }
    ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
        int lenA = getListLength(headA);
        int lenB = getListLength(headB);
        ListNode* common = NULL,*startA=headA,*startB=headB;
        if(lenA<lenB){
            int diff = lenB-lenA;
            while(diff--) startB=startB->next;
        }else{
            int diff = lenA-lenB;
            while(diff--) startA=startA->next;
        }
        while(startA){
            if(startA == startB){
                common = startA;
                break;
            }
            startA=startA->next;
            startB=startB->next;
        }
        return common;
    }
};

Next challenges: (M) Swap Nodes in Pairs (H) Copy List with Random Pointer (M) Insertion Sort List