Given a string find the character which occurs a minimum number of times

Given a string find the character which occurs a minimum number of times.

Note: If multiple such characters exist output anyone.

Example:

Input: S = abbccdd

Output: a

As frequency of every element is more than that of a

Input: S = abccdd

Output: b or Output: a

Both the outputs are acceptable according to given conditions.

Naive Approach:

We can run nested loops and keep the counter for a minimum frequency and can compare it with the frequency of each element found in one pass of for loop.

Complexity: O(n²)

Better Approach:

As there are only 26 characters in the English language we can safely assume the complexity of unordered_map in c++ or any Hash Table function to be O(1) in case searching.

We may even use a frequency array of size 26 in this approach instead of unordered_map

//Programm to find character occuring minimum number of times.
#include<bits/stdc++.h>
using namespace std;
char minFreq(string s)
{
  unordered_map<char,int>freq; // for counting frequency of each character
  int min=INT_MAX; 
  char ans; // this is to be returned st last
  for(auto x:s) // Counting frequency of each character
    freq[x]++;
  for(auto x:freq) //Finding Character With MIN FREQ
  {
    if(x.second<min)
    {
      min=x.second;
      ans=x.first;
    }
  }
  return ans;
}
int main() {
    
    string s="Cplusplus";
    cout<<"Character With MIN FREQ :"<<minFreq(s);
    return 0;
}

Output

Character With MIN FREQ :C