Problem link : 1920D - Повторение в массиве Submission : 248332727 My Approach : I stored the sizes of the array in a vector whenever an element is inserted and stored that element in a map. Incase of array repititon i just updated the size. To find the number at a given index im using a upper_bound() and iterating until it points to an index present in the map. This approach is worked well for many cases. But im getting wrong answer. Im unable to figure out where it is going wrong. can someone please help me with this .