Finding the Middle Node

Procedure to find the middle node of a singly linked list.

First Method : Finding Length & Then Middle Element

  • In the first scan of the linked list, find the length of linked list.
  • Find the ceiling value of length/2.
  • Traverse to the node having that index.
int findMid(struct node *ptr) {
​
int len = 0;
// Finding the length of linked list
while (ptr != NULL) {
len++;
ptr = ptr -> next;
}
// Find the middle position
int mid = ceil(len/2);
// Resetting ptr to head
ptr = head;
// Return the middle element
for (int i = 0; i < mid; i++) {
ptr = ptr -> next;
}
return ptr -> data;
​
}

Second Method : Using Two Pointers

  • Take two pointers with initial value as the head address.
  • Move pointer 2 two times in each iteration while it is not NULL.
  • If pointer 2 is not NULL, move pointer 1 once.
int findMid() {
​
// Two pointers
struct node *ptr1 = head;
struct node *ptr2 = head;
// Move until pointer 2 becomes NULL
while (ptr2 != NULL) {
// Move pointer 2 two times
ptr2 = ptr2 -> next;
// Don't move again if already NULL
if (ptr2 != NULL) {
ptr2 = ptr2 -> next;
}
// Move pointer 1 if pointer 2 is not NULL
if (ptr2 != NULL) {
ptr1 = ptr1 -> next;
}
}
return ptr1 -> data;
​
}
Contributed by Nitin Ranganath