Binary Search on Singly Linked List in C++

A singly linked list is a linked list (a data structure that stores a node’s value and the memory location of the next node) which can go only one way.

A binary search is a search algorithm based on divide and rule. That finds the middle element of the structure and compares and uses recursive calls to the same algorithm for inequality.

Here, we are given a singly linked list and an element to be found using a binary search.

Since the singly linked list is a data structure that uses only one pointer, it is not easy to find its middle element. To mid of singly linked list, we use two pointer approaches.

ALGORITHM

Step 1 : Initialize, start_node (head of list) and last_node (will have last value) , mid_node (middle node of the structure).
Step 2 : Compare mid_node to element
   Step 2.1 : if mid_node = element, return value “found”.
   Step 2.2 : if mid_node > element, call binary search on lower_Half.
   Step 2.3 : if mid_node < element, call binary search on upper_Half.
Step 3 : if entire list is traversed, return “Not found”.

Example

Live Demo

#include<stdio.h>
#include<stdlib.h>
struct Node{
   int data;
   struct Node* next;
};
Node *newNode(int x){
   struct Node* temp = new Node;
   temp->data = x;
   temp->next = NULL;
   return temp;
}
struct Node* mid_node(Node* start, Node* last){
   if (start == NULL)
      return NULL;
   struct Node* slow = start;
   struct Node* fast = start -> next;
   while (fast != last){
      fast = fast -> next;
      if (fast != last){
         slow = slow -> next;
         fast = fast -> next;
      }
   }
   return slow;
}
struct Node* binarySearch(Node *head, int value){
   struct Node* start = head;
   struct Node* last = NULL;
   do{
      Node* mid = mid_node(start, last);
      if (mid == NULL)
         return NULL;
      if (mid -> data == value)
         return mid;
      else if (mid -> data < value)
         start = mid -> next;
      else
         last = mid;
   }
   while (last == NULL || last != start);
      return NULL;
}
int main(){
   Node *head = newNode(54);
   head->next = newNode(12);
   head->next->next = newNode(18);
   head->next->next->next = newNode(23);
   head->next->next->next->next = newNode(52);
   head->next->next->next->next->next = newNode(76);
   int value = 52;
   if (binarySearch(head, value) == NULL)
      printf("Value is not present in linked list\n");
   else
      printf("The value is present in linked list\n");
   return 0;
}