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Remove duplicates from sorted array

  • Difficulty Level : Easy
  • Last Updated : 15 Nov, 2021

Given a sorted array, the task is to remove the duplicate elements from the array.
Examples: 
 

Input  : arr[] = {2, 2, 2, 2, 2}
Output : arr[] = {2}
         new size = 1

Input  : arr[] = {1, 2, 2, 3, 4, 4, 4, 5, 5}
Output : arr[] = {1, 2, 3, 4, 5}
         new size = 5

 

Method 1: (Using extra space) 
 

  1. Create an auxiliary array temp[] to store unique elements.
  2. Traverse input array and one by one copy unique elements of arr[] to temp[]. Also keep track of count of unique elements. Let this count be j.
  3. Copy j elements from temp[] to arr[] and return j

 

C++




// Simple C++ program to remove duplicates
#include<iostream>
using namespace std;
 
// Function to remove duplicate elements
// This function returns new size of modified
// array.
int removeDuplicates(int arr[], int n)
{
    // Return, if array is empty
    // or contains a single element
    if (n==0 || n==1)
        return n;
 
    int temp[n];
 
    // Start traversing elements
    int j = 0;
    for (int i=0; i<n-1; i++)
 
        // If current element is not equal
        // to next element then store that
        // current element
        if (arr[i] != arr[i+1])
            temp[j++] = arr[i];
 
    // Store the last element as whether
    // it is unique or repeated, it hasn't
    // stored previously
    temp[j++] = arr[n-1];
 
    // Modify original array
    for (int i=0; i<j; i++)
        arr[i] = temp[i];
 
    return j;
}
 
// Driver code
int main()
{
    int arr[] = {1, 2, 2, 3, 4, 4, 4, 5, 5};
    int n = sizeof(arr) / sizeof(arr[0]);
 
    // removeDuplicates() returns new size of
    // array.
    n = removeDuplicates(arr, n);
 
    // Print updated array
    for (int i=0; i<n; i++)
       cout << arr[i] << " ";
 
    return 0;
}
Java



// simple java program to remove
// duplicates
 
class Main
{
    // Function to remove duplicate elements
    // This function returns new size of modified
    // array.
    static int removeDuplicates(int arr[], int n)
    {
        // Return, if array is empty
        // or contains a single element
        if (n==0 || n==1)
            return n;
      
        int[] temp = new int[n];
         
        // Start traversing elements
        int j = 0;
        for (int i=0; i<n-1; i++)
            // If current element is not equal
            // to next element then store that
            // current element
            if (arr[i] != arr[i+1])
                temp[j++] = arr[i];
         
        // Store the last element as whether
        // it is unique or repeated, it hasn't
        // stored previously
        temp[j++] = arr[n-1];  
         
        // Modify original array
        for (int i=0; i<j; i++)
            arr[i] = temp[i];
      
        return j;
    }
     
    public static void main (String[] args)
    {
        int arr[] = {1, 2, 2, 3, 4, 4, 4, 5, 5};
        int n = arr.length;
         
        n = removeDuplicates(arr, n);
  
        // Print updated array
        for (int i=0; i<n; i++)
           System.out.print(arr[i]+" ");
    }
}
Python3



# Python3 program to
# remove duplicates
# Function to remove
# duplicate elements
 
# This function returns
# new size of modified
# array.
def removeDuplicates(arr, n):
 
    # Return, if array is
    # empty or contains
    # a single element
    if n == 0 or n == 1:
        return n
 
    temp = list(range(n))
 
    # Start traversing elements
    j = 0;
    for i in range(0, n-1):
 
        # If current element is
        # not equal to next
        # element then store that
        # current element
        if arr[i] != arr[i+1]:
            temp[j] = arr[i]
            j += 1
 
    # Store the last element
    # as whether it is unique
    # or repeated, it hasn't
    # stored previously
    temp[j] = arr[n-1]
    j += 1
     
    # Modify original array
    for i in range(0, j):
        arr[i] = temp[i]
 
    return j
 
# Driver code
arr = [1, 2, 2, 3, 4, 4, 4, 5, 5]
n = len(arr)
 
# removeDuplicates() returns
# new size of array.
n = removeDuplicates(arr, n)
 
# Print updated array
for i in range(n):
    print ("%d"%(arr[i]), end = " ")
C#



// Simple C# program to remove
// duplicates
using System;
 
class GFG {
     
    // Function to remove duplicate
    // elements This function returns
    // new size of modified array.
    static int removeDuplicates(int []arr, int n)
    {
         
        // Return, if array is empty
        // or contains a single element
        if (n == 0 || n == 1)
            return n;
     
        int []temp = new int[n];
         
        // Start traversing elements
        int j = 0;
         
        for (int i = 0; i < n - 1; i++)
         
            // If current element is not equal
            // to next element then store that
            // current element
            if (arr[i] != arr[i+1])
                temp[j++] = arr[i];
         
        // Store the last element as
        // whether it is unique or
        // repeated, it hasn't
        // stored previously
        temp[j++] = arr[n-1];
         
        // Modify original array
        for (int i = 0; i < j; i++)
            arr[i] = temp[i];
     
        return j;
    }
     
    public static void Main ()
    {
        int []arr = {1, 2, 2, 3, 4, 4, 4, 5, 5};
        int n = arr.Length;
         
        n = removeDuplicates(arr, n);
 
        // Print updated array
        for (int i = 0; i < n; i++)
            Console.Write(arr[i] + " ");
    }
}
 
// This code is contributed by nitin mittal.
Javascript



<script>
 
// Simple JavaScript program to remove
// duplicates
 
// Function to remove duplicate elements
// This function returns new size of modified
// array.
function removeDuplicates(arr, n)
{
    // Return, if array is empty
    // or contains a single element
    if (n==0 || n==1)
        return n;
 
    var temp = new Array(n);
 
    // Start traversing elements
    var j = 0;
    for (var i=0; i<n-1; i++)
 
        // If current element is not equal
        // to next element then store that
        // current element
        if (arr[i] != arr[i+1])
            temp[j++] = arr[i];
 
    // Store the last element as whether
    // it is unique or repeated, it hasn't
    // stored previously
    temp[j++] = arr[n-1];
 
    // Modify original array
    for (var i=0; i<j; i++)
        arr[i] = temp[i];
 
    return j;
}
 
var arr = [1, 2, 2, 3, 4, 4, 4, 5, 5];
    var n = arr.length;
 
    // removeDuplicates() returns new size of
    // array.
    n = removeDuplicates(arr, n);
 
    // Print updated array
    for (var i=0; i<n; i++)
       document.write( arr[i]+" ");
 
// This code is contributed by SoumikMondal
 
</script>
Output:  
1 2 3 4 5
Time Complexity : O(n) 
Auxiliary Space : O(n)
Method 2: (Constant extra space) 
Just maintain a separate index for same array as maintained for different array in Method 1.
 
C++



// C++ program to remove duplicates in-place
#include<iostream>
using namespace std;
 
// Function to remove duplicate elements
// This function returns new size of modified
// array.
int removeDuplicates(int arr[], int n)
{
    if (n==0 || n==1)
        return n;
 
    // To store index of next unique element
    int j = 0;
 
    // Doing same as done in Method 1
    // Just maintaining another updated index i.e. j
    for (int i=0; i < n-1; i++)
        if (arr[i] != arr[i+1])
            arr[j++] = arr[i];
 
    arr[j++] = arr[n-1];
 
    return j;
}
 
// Driver code
int main()
{
    int arr[] = {1, 2, 2, 3, 4, 4, 4, 5, 5};
    int n = sizeof(arr) / sizeof(arr[0]);
 
    // removeDuplicates() returns new size of
    // array.
    n = removeDuplicates(arr, n);
 
    // Print updated array
    for (int i=0; i<n; i++)
        cout << arr[i] << " ";
 
    return 0;
}
Java



// simple java program to remove
// duplicates
 
class Main
{
    // Function to remove duplicate elements
    // This function returns new size of modified
    // array.
    static int removeDuplicates(int arr[], int n)
    {
        if (n == 0 || n == 1)
            return n;
      
        // To store index of next unique element
        int j = 0;
      
        // Doing same as done in Method 1
        // Just maintaining another updated index i.e. j
        for (int i = 0; i < n-1; i++)
            if (arr[i] != arr[i+1])
                arr[j++] = arr[i];
      
        arr[j++] = arr[n-1];
      
        return j;
    }
     
    public static void main (String[] args)
    {
        int arr[] = {1, 2, 2, 3, 4, 4, 4, 5, 5};
        int n = arr.length;
         
        n = removeDuplicates(arr, n);
  
        // Print updated array
        for (int i=0; i<n; i++)
           System.out.print(arr[i]+" ");
    }
}
 
/* This code is contributed by Harsh Agarwal */
Python3 

# Python3 program to remove
# duplicate elements

# This function returns new 
# size of modified array
def removeDuplicates(arr, n):
    if n == 0 or n == 1:
        return n

    # To store index of next
    # unique element
    j = 0

    # Doing same as done
    # in Method 1 Just
    # maintaining another 
    # updated index i.e. j
    for i in range(0, n-1):
        if arr[i] != arr[i+1]:
            arr[j] = arr[i]
            j += 1

    arr[j] = arr[n-1]
    j += 1
    return j

# Driver code
arr = [1, 2, 2, 3, 4, 4, 4, 5, 5]
n = len(arr)

# removeDuplicates() returns
# new size of array.
n = removeDuplicates(arr, n)

# Print updated array
for i in range(0, n):
    print (" %d "%(arr[i]), end = " ")


C#



// simple C# program to remove
// duplicates
using System;
 
class GfG {
     
    // Function to remove duplicate
    // elements This function returns
    // new size of modified array.
    static int removeDuplicates(int []arr, int n)
    {
         
        if (n == 0 || n == 1)
            return n;
     
        // To store index of next
        // unique element
        int j = 0;
     
        // Doing same as done in Method 1
        // Just maintaining another updated
        // index i.e. j
        for (int i = 0; i < n - 1; i++)
            if (arr[i] != arr[i + 1])
                arr[j++] = arr[i];
     
        arr[j++] = arr[n - 1];
     
        return j;
    }
     
    public static void Main ()
    {
        int []arr = {1, 2, 2, 3, 4, 4,
                                 4, 5, 5};
        int n = arr.Length;
         
        n = removeDuplicates(arr, n);
 
        // Print updated array
        for (int i = 0; i < n; i++)
            Console.Write(arr[i] + " ");
    }
}
 
// This code is contributed by parashar.
Javascript



<script>
// simple javascript program to remove
// duplicates
 
    // Function to remove duplicate elements
    // This function returns new size of modified
    // array.
    function removeDuplicates(arr , n) {
        if (n == 0 || n == 1)
            return n;
 
        // To store index of next unique element
        var j = 0;
 
        // Doing same as done in Method 1
        // Just maintaining another updated index i.e. j
        for (i = 0; i < n - 1; i++)
            if (arr[i] != arr[i + 1])
                arr[j++] = arr[i];
 
        arr[j++] = arr[n - 1];
 
        return j;
    }
 
     
        var arr = [ 1, 2, 2, 3, 4, 4, 4, 5, 5 ];
        var n = arr.length;
 
        n = removeDuplicates(arr, n);
 
        // Print updated array
        for (i = 0; i < n; i++)
            document.write(arr[i] + " ");
 
// This code is contributed by umadevi9616
</script>
Output: 
 
1 2 3 4 5
Time Complexity : O(n) 
Auxiliary Space : O(1)
This article is contributed by Sahil Chhabra. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
 

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