Check if two linked lists merge. If so, where?

Question

This question may be old, but I couldn\'t think of an answer.

Say, there are two lists of different lengths, merging at a point; how do we know wher

Answer 1

Here is naive solution , No neeed to traverse whole lists.

if your structured node has three fields like

struct node {
    int data;   
    int flag;  //initially set the flag to zero  for all nodes
    struct node *next;
};

say you have two heads (head1 and head2) pointing to head of two lists.

Traverse both the list at same pace and put the flag =1(visited flag) for that node ,

  if (node->next->field==1)//possibly longer list will have this opportunity
      //this will be your required node. 

Answer 2

A O(n) complexity solution. But based on an assumption.

assumption is: both nodes are having only positive integers.

logic : make all the integer of list1 to negative. Then walk through the list2, till you get a negative integer. Once found => take it, change the sign back to positive and return.

static int findMergeNode(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {

    SinglyLinkedListNode current = head1; //head1 is give to be not null.

    //mark all head1 nodes as negative
    while(true){
        current.data = -current.data;
        current = current.next;
        if(current==null) break;
    }

    current=head2; //given as not null
    while(true){
        if(current.data<0) return -current.data;
        current = current.next;
    }

}