Arrays n Hashing

Question 1

Find two repeated

Answer 1

The trick here is to modify the current array by using element as index in range 0 to n-1 and multiply by -1 to keep track of element. Use arr[abs(arr[i])] *= -1

Question 2

Hashing for pair-2 sum

Answer 2

In case of distinct elements, we can just use a set. In case duplicates are present, store count of elements in the map.