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Find the first repeated element in an array by its index

Objective: Given an array of integers, find out the first repeated element. First repeated element means the element occurs atleast twice and has smallest index.

Input: An Array

Output: The first repeated element

Examples :

Array {1,1,3,4,6,7,9} first repeated Number : 1
Array {7,2,2,3,7} first repeated Number : 7
Array {5,3,3,3,3,3,3,3,4,4,4,5,3} first repeated Number : 5


Naive Solution :

Use two for loops. Time Complexity O(N2).

Better Solution: Using HastSet, Time Complexity O(N).

  • Take a variable say index = -1.
  • Initialize a HashSet.
  • Navigate the array from right to left(backwards) taking one element at a time
  • if HashSet doesn't contain the element, add it
  • if HashSet contains then update the index with the current index in navigation.
  • At the end index will be updated by the element which is repeated and has the lowest index.



first repeated element by index is : 2

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