Detect loop in a linked list | C ++ program to return first node of loop ; A utility function to print a linked list ; returns distance between first and last node every time * last node moves forwars ; counts no of nodes between first and last ; Function to detect first node of loop in a linked list that may contain loop ; Create a temporary node ; first always points to head ; last pointer initially points to head ; current_length stores no of nodes between current * position of first and last ; current_length stores no of nodes between previous * position of first and last ; set prev_length to current length then update the current length ; distance is calculated ; last node points the next node ; Driver program to test above function ; Create a loop for testing ( 5 is pointing to 3 )
#include <bits/stdc++.h> NEW_LINE using namespace std ; struct Node { int key ; struct Node * next ; } ; Node * newNode ( int key ) { Node * temp = new Node ; temp -> key = key ; temp -> next = NULL ; return temp ; } void printList ( Node * head ) { while ( head != NULL ) { cout << head -> key << " ▁ " ; head = head -> next ; } cout << endl ; } int distance ( Node * first , Node * last ) { int counter = 0 ; Node * curr ; curr = first ; while ( curr != last ) { counter += 1 ; curr = curr -> next ; } return counter + 1 ; } bool detectLoop ( Node * head ) { Node * temp = new Node ; Node * first , * last ; first = head ; last = head ; int current_length = 0 ; int prev_length = -1 ; while ( current_length > prev_length && last != NULL ) { prev_length = current_length ; current_length = distance ( first , last ) ; last = last -> next ; } if ( last == NULL ) { return false ; } else { return true ; } } int main ( ) { Node * head = newNode ( 1 ) ; head -> next = newNode ( 2 ) ; head -> next -> next = newNode ( 3 ) ; head -> next -> next -> next = newNode ( 4 ) ; head -> next -> next -> next -> next = newNode ( 5 ) ; head -> next -> next -> next -> next -> next = head -> next -> next ; bool found = detectLoop ( head ) ; if ( found ) cout << " Loop ▁ Found " ; else cout << " No ▁ Loop ▁ Found " ; return 0 ; }