Given an array of N integers. Each element in the array occurs same number of times except one element. The task is to find this element.

**Examples:**

Input :arr[] = {1, 1, 2, 2, 3}Output :3Input :arr[] = {0, 1, 2, 4, 4}Output :4

The idea is to use a hash table **freq** to store the frequencies of given elements. Once we have frequencies in the hash table, we can traverse the table to find the only value which is different from others.

Below is the implementation of the above idea :

`// C++ program to find the element having ` `// different frequency than other array ` `// elements having same frequency ` `#include <bits/stdc++.h> ` `using` `namespace` `std; `
` ` `// Function to find the element having ` `// different frequency from other array ` `// elements with same frequency ` `int` `findElement(` `int` `arr[], ` `int` `n) `
`{ ` ` ` `// Store frequencies of elements `
` ` `unordered_map<` `int` `, ` `int` `> freq; `
` ` `for` `(` `int` `i = 0; i < n; i++) { `
` ` ` ` `// increase the value by 1 for every `
` ` `// time the element occurs in an array `
` ` `freq[arr[i]]++; `
` ` `} `
` ` ` ` `// Below code is used find the only different `
` ` `// value in freq. `
` ` `auto` `it = freq.begin(); `
` ` `int` `fst_fre = it->second, fst_ele = it->first; `
` ` `if` `(freq.size() <= 2) `
` ` `return` `fst_fre; `
` ` `it++; `
` ` `int` `sec_fre = it->second, sec_ele = it->first; `
` ` `it++; `
` ` `int` `trd_fre = it->second, trd_ele = it->first; `
` ` `if` `(sec_fre == fst_fre && sec_fre != trd_fre) `
` ` `return` `trd_ele; `
` ` `if` `(sec_fre == trd_fre && sec_fre != fst_fre) `
` ` `return` `fst_ele; `
` ` `if` `(fst_fre == trd_fre && sec_fre != fst_fre) `
` ` `return` `sec_ele; `
` ` ` ` `// We reach here when first three frequencies are same `
` ` `it++; `
` ` `for` `(; it != freq.end(); it++) { `
` ` `if` `(it->second != fst_fre) `
` ` `return` `it->first; `
` ` `} `
` ` ` ` `return` `-1; `
`} ` ` ` `// Driver code ` `int` `main() `
`{ ` ` ` `int` `arr[] = { 0, 1, 2, 4, 4 }; `
` ` `int` `n = ` `sizeof` `(arr) / ` `sizeof` `(arr[0]); `
` ` `cout << findElement(arr, n) << endl; `
` ` `return` `0; `
`} ` |

*chevron_right*

*filter_none*

`# Python program to find the element having ` `# different frequency than other array ` `# elements having same frequency ` ` ` `# Function for above implementation ` `def` `findElement(arr, n) : `
` ` ` ` `# Empty dictionary to hold the values `
` ` `freq ` `=` `{} `
` ` ` ` `# Initialization of frequencies of each `
` ` `# element to 0 `
` ` `for` `i ` `in` `range` `(` `0` `, n) : `
` ` ` ` `freq[arr[i]] ` `=` `0`
` ` ` ` `# Count of frequencies of elements `
` ` `for` `i ` `in` `range` `(` `0` `, n) : `
` ` ` ` `freq[arr[i]] ` `=` `freq[arr[i]] ` `+` `1`
` ` ` ` `# Storing the first value of dictionary `
` ` `trd_ele ` `=` `freq[` `0` `] `
` ` ` ` `# Variable to hold the final result `
` ` `position ` `=` `-` `1`
` ` ` ` `# Following loop iterates through the dictionary `
` ` `# and checks if frequencies are different `
` ` `# from the frequency of the first element `
` ` `for` `i ` `in` `freq : `
` ` ` ` `flag ` `=` `freq[i] `
` ` ` ` `if` `trd_ele !` `=` `flag : `
` ` ` ` `# Difference has been detected `
` ` `position ` `=` `i `
` ` `break`
` ` ` ` `# Following lines of code checks if the first `
` ` `# element is itself the required anomaly by `
` ` `# comparing the frequencies of first 3 elements `
` ` `fst_ele ` `=` `freq[` `1` `] `
` ` `sec_ele ` `=` `freq[` `2` `] `
` ` ` ` `if` `trd_ele !` `=` `fst_ele : `
` ` ` ` `if` `trd_ele !` `=` `sec_ele : `
` ` ` ` `for` `i ` `in` `freq : `
` ` ` ` `# First element is the desired result `
` ` `position ` `=` `i `
` ` `break`
` ` ` ` `# Final result is returned `
` ` `return` `position `
` ` ` ` `# Driver code ` `arr ` `=` `[ ` `0` `, ` `1` `, ` `2` `, ` `4` `, ` `4` `] `
`# Variable to store length of array ` `n ` `=` `len` `(arr) `
`print` `(findElement(arr, n)) `
` ` `# This code is contributed by Pratik Basu ` |

*chevron_right*

*filter_none*

**Output:**

4

Time Complexity : O(n)

Auxiliary Space : O(n)

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the **DSA Self Paced Course** at a student-friendly price and become industry ready.

## Recommended Posts:

- Smallest subarray having an element with frequency greater than that of other elements
- Maximum difference between frequency of two elements such that element having greater frequency is also greater
- Find element in array with frequency equal to sum of frequencies of other elements
- Check if frequency of character in one string is a factor or multiple of frequency of same character in other string
- Check if minimum element in array is less than or equals half of every other element
- Length of longest subarray in which elements greater than K are more than elements not greater than K
- Find frequency of each element in a limited range array in less than O(n) time
- Find element in a sorted array whose frequency is greater than or equal to n/2.
- Find the maximum element in the array other than A
_{i} - Longest sub-string having frequency of each character less than equal to k
- Number of positions such that adding K to the element is greater than sum of all other elements
- Longest subsequence with first and last element greater than all other elements
- Count of pairs having each element equal to index of the other from an Array
- Count of elements whose absolute difference with the sum of all the other elements is greater than k
- Sum of elements in an array having prime frequency
- Product of elements in an array having prime frequency
- XOR of elements in an array having prime frequency
- Sum of elements in an array having composite frequency
- Largest Divisor for each element in an array other than 1 and the number itself
- Find the element that appears once in an array where every other element appears twice

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.