Last duplicate element in a sorted array

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We have a sorted array with duplicate elements and we have to find the index of last duplicate element and print index of it and also print the duplicate element. If no such element found print a message.

Examples:

Input : arr[] = {1, 5, 5, 6, 6, 7}
Output :
Last index: 4
Last duplicate item: 6

Input : arr[] = {1, 2, 3, 4, 5}
Output : No duplicate found

We simply iterate through the array in reverse order and compare the current and previous element. If a match is found then we print the index and duplicate element. As this is sorted array it will be the last duplicate. If no such element is found we will print the message for it.

for i = n-1 to 0
if (arr[i] == arr[i-1])
Print current element and its index.
Return

If no such element found print a message of no duplicate found.

Implementation:

C++

// To print last duplicate element and its
// index in a sorted array
#include <bits/stdc++.h>
 
void dupLastIndex(int arr[], int n) {
 
  // if array is null or size is less 
  // than equal to 0 return
  if (arr == NULL || n <= 0) 
    return;
   
  // compare elements and return last
  // duplicate and its index
  for (int i = n - 1; i > 0; i--) {
    if (arr[i] == arr[i - 1]) {
      printf("Last index: %d\nLast "
            "duplicate item: %d\n", i, arr[i]);
      return;
    }
  }
 
  // If we reach here, then no duplicate
  // found.
  printf("no duplicate found");
}
 
int main() {
  int arr[] = {1, 5, 5, 6, 6, 7, 9};
  int n = sizeof(arr) / sizeof(int);
  dupLastIndex(arr, n);
  return 0;
}

C

// C code To print last duplicate element and its
// index in a sorted array
#include <stdio.h>
 
void dupLastIndex(int arr[], int n)
{
 
    // if array is null or size is less
    // than equal to 0 return
    if (arr == NULL || n <= 0)
        return;
 
    // compare elements and return last
    // duplicate and its index
    for (int i = n - 1; i > 0; i--) {
        if (arr[i] == arr[i - 1]) {
            printf("Last index: %d\nLast "
                   "duplicate item: %d\n",
                   i, arr[i]);
            return;
        }
    }
 
    // If we reach here, then no duplicate
    // found.
    printf("no duplicate found");
}
 
int main()
{
    int arr[] = { 1, 5, 5, 6, 6, 7, 9 };
    int n = sizeof(arr) / sizeof(int);
    dupLastIndex(arr, n);
    return 0;
}
 
// This code is contributed by muditj148.

Java

// Java code to print last duplicate element 
// and its index in a sorted array
import java.io.*;
 
class GFG
{
    static void dupLastIndex(int arr[], int n) 
    {
        // if array is null or size is less 
        // than equal to 0 return
        if (arr == null || n <= 0) 
            return;
         
        // compare elements and return last
        // duplicate and its index
        for (int i = n - 1; i > 0; i--) 
        {
            if (arr[i] == arr[i - 1]) 
            {
            System.out.println("Last index:" + i);
            System.out.println("Last duplicate item: "
                              + arr[i]);
            return;
            }
        }
         
        // If we reach here, then no duplicate
        // found.
        System.out.print("no duplicate found");
    }
 
    // Driver code 
    public static void main (String[] args) 
    {
        int arr[] = {1, 5, 5, 6, 6, 7, 9};
        int n = arr.length;
        dupLastIndex(arr, n);
         
    }
}
 
// This code is contributed by vt_m

Python3

# Python3 code to print last duplicate 
# element and its index in a sorted array
 
def dupLastIndex(arr, n): 
 
    # if array is null or size is less 
    # than equal to 0 return
    if (arr == None or n <= 0): 
        return
 
    # compare elements and return last
    # duplicate and its index
    for i in range(n - 1, 0, -1): 
         
        if (arr[i] == arr[i - 1]): 
            print("Last index:", i, "\nLast",
                     "duplicate item:",arr[i])
            return
         
    # If we reach here, then no duplicate
    # found.
    print("no duplicate found")
     
 
arr = [1, 5, 5, 6, 6, 7, 9]
n = len(arr) 
dupLastIndex(arr, n)
 
# This code is contributed by 
# Smitha Dinesh Semwal

C#

// C# code to print last duplicate element 
// and its index in a sorted array
using System;
 
class GFG {
     
    static void dupLastIndex(int []arr, int n) 
    {
         
        // if array is null or size is less 
        // than equal to 0 return
        if (arr == null || n <= 0) 
            return;
         
        // compare elements and return last
        // duplicate and its index
        for (int i = n - 1; i > 0; i--) 
        {
            if (arr[i] == arr[i - 1]) 
            {
                Console.WriteLine("Last index:" + i);
                Console.WriteLine("Last duplicate item: "
                                + arr[i]);
                return;
            }
        }
         
        // If we reach here, then no duplicate
        // found.
        Console.WriteLine("no duplicate found");
    }
 
    // Driver code 
    public static void Main () 
    {
        int []arr = {1, 5, 5, 6, 6, 7, 9};
        int n = arr.Length;
         
        dupLastIndex(arr, n);
    }
}
 
// This code is contributed by vt_m.

PHP

<?php
// PHP program to print last 
// duplicate element and its
// index in a sorted array
 
function dupLastIndex($arr, $n)
{
     
    // if array is null or size is less 
    // than equal to 0 return
    if ($arr == null or $n <= 0) 
        return;
     
    // compare elements and return last
    // duplicate and its index
    for ( $i = $n - 1; $i > 0; $i--)
    {
        if ($arr[$i] == $arr[$i - 1])
        {
            echo "Last index:", $i , "\n";
            echo "Last duplicate item:", $arr[$i];
            return;
    }
}
 
    // If we reach here, then
    // no duplicate found.
    echo "no duplicate found";
}
 
// Driver Code
$arr = array(1, 5, 5, 6, 6, 7, 9);
$n = count($arr);
dupLastIndex($arr, $n);
 
// This code is contributed by anuj_67.
?>

Javascript

<script>
 
// JavaScript program to print last duplicate element 
// and its index in a sorted array
 
    function dupLastIndex(arr, n) 
    {
        // if array is null or size is less 
        // than equal to 0 return
        if (arr == null || n <= 0) 
            return;
           
        // compare elements and return last
        // duplicate and its index
        for (let i = n - 1; i > 0; i--) 
        {
            if (arr[i] == arr[i - 1]) 
            {
            document.write("Last index:" + i + "<br/>");
            document.write("Last duplicate item: "
                              + arr[i] + "<br/>");
            return;
            }
        }
           
        // If we reach here, then no duplicate
        // found.
        document.write("no duplicate found");
    }
 
  
// Driver code
 
        let arr = [1, 5, 5, 6, 6, 7, 9];
        let n = arr.length;
        dupLastIndex(arr, n);
 
</script>

Output:

Last index: 4
Last duplicate item: 6

Time Complexity: O(n)
Auxiliary Space: O(1), as no extra space is used.

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