Find duplicate number in array

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Brute Force Approach

Sort the given array and run a loop to find the duplicate number.

TC – O(nlogn)  |  SC – O(1)

Better Approach

Using HashMap

  • Initialize an empty hashmap.
  • Iterate over the elements of the array.
  • For each element, increase its occurrence count in the hashmap by 1.
  • Iterate over the elements of the hashmap.
  • For each element in the hashmap, check if its occurrence count is greater than 1. If it is, print the element as a duplicate.
  • If no duplicates are found, print a message indicating this.

TC – O(n)

SC – O(n)

Optimal Approach

Using Floyd Cycle detection

We will take two pointers one will move fast and one slow.

using namespace std;

int findDuplicates(int arr[], int arrSize){
	int fast = 0,
      	slow = 0;
     	slow = arr[slow];
        fast = arr[arr[fast]];
    } while(slow != fast);

  	slow = 0;
  	while(slow != fast){
		slow = arr[slow];
        fast = arr[fast];
  	return slow; //or fast

int main(){
  	int arrWithDuplicates[] = {2,5,1,1,4,3};
  	int arrSize = sizeof(arrWithDuplicates)/sizeof(arrWithDuplicates[0]);
  	cout<<findDuplicates(arrWithDuplicates, arrSize);
	return 0;



TC – O(n)

SC – O(1)

Find duplicate number in array