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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 doesnt con­tain ele­ment, add it
  • if Hash­Set con­tains then update the index with cur­rent index in navigation.
  • At the end index will be updated by the ele­ment which is repeated and has the low­est index.

Com­plete Code:


first repeated element by index is : 2

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  • Bill

    Code is wrong
    a)left paren­the­sis miss­ing in System.out.println”{1,2,5,7,5,3,10,2}”);
    b) for loop needs to stop at i >=0 else it never enters as it is

    • tuto­ri­al­hori­zon

      Thanks bill. Cor­rected it.