First non-repeating character using one traversal of string | Set 2

// CPP program to find first non-repeating
// character using 1D array and one traversal.
#include <bits/stdc++.h>
using namespace std;
#define NO_OF_CHARS 256

/* The function returns index of the first
non-repeating character in a string. If
all characters are repeating then
returns INT_MAX */
int firstNonRepeating(char* str)
{
  
    // Initialize all characters as
    // absent.
    int arr[NO_OF_CHARS];
    for (int i = 0; i < NO_OF_CHARS; i++)
        arr[i] = -1;

    // After below loop, the value of
    // arr[x] is going to be index of
    // of x if x appears only once. Else
    // the value is going to be either
    // -1 or -2.
    for (int i = 0; str[i]; i++) {
        if (arr[str[i]] == -1)
            arr[str[i]] = i;
        else
            arr[str[i]] = -2;
    }

    int res = INT_MAX;
    for (int i = 0; i < NO_OF_CHARS; i++)

        // If this character occurs only
        // once and appears before the
        // current result, then update the
        // result
        if (arr[i] >= 0)
            res = min(res, arr[i]);

    return res;
}

/* Driver program to test above function */
int main()
{
    char str[] = "geeksforgeeks";
    int index = firstNonRepeating(str);
    if (index == INT_MAX)
        cout <<"Either all characters are "
               "repeating or string is empty";
    else
        cout <<"First non-repeating character"
               " is "<< str[index];
    return 0;
}

// This code is contributed by shivanisinghss210

// C++ implementation of
// above approach
#include <bits/stdc++.h> 
#include <unordered_map> 
using namespace std; 

// Function to make a frequency
// dictionary of characters in a string.
unordered_map<char, int> makeHashMap(string string)
{
    // Create an empty HashMap
    unordered_map<char, int> d1;
 
    // To store each character and its frequency
    char c;
 
    // Traversing through the string
    for (int i = 0; i < string.length(); i++) {
 
        // Extracting each character
        c = string[i];
 
        // If the character has occurred before
        if (d1.find(c) != d1.end()) {
 
            // Increment its frequency
            d1[c]++;
        }
 
        // If the character has not occurred before
        else
            d1[c] = 1;
    }
 
    // Update value of each key such that
    // if frequency of  frequency of  a key
    // is equal to 1, then it is set to 0,
    // else set the value equal to None
    for (auto it = d1.begin(); it != d1.end(); it++)
    {
        if (it->second == 1)
            it->second = 0;
        else
            it->second = -1;
    }
 
    return d1;
}
 
// Function to return the first
// non repeating character
char firstNotRepeatingCharacter(string s)
{
    // Make a frequency dictionary
    unordered_map<char, int> d = makeHashMap(s);
 
    // Variable to store the result
    char nonRep = '.';
 
    // Traversing through the string only once
    for (int i = 0; i < s.length(); i++) {
        char c = s[i];
 
        // If the character's value in the
        // frequency dictionary is "not None"
        if (d[c] == 0) {
 
            // Store it in the result
            nonRep = c;
 
            // Break out of the loop
            break;
        }
    }
 
    // If no non-repeating character is found
    if (nonRep == '.')
        return '_';
    else
        return nonRep;
}
 
// Driver Code
int main()
{
    string s = "bbcdcca";
    cout << firstNotRepeatingCharacter(s);
    return 0;
}

#include<bits/stdc++.h>
using namespace std;

// C++ implementation of above approach

void firstNonRepeating(string s)
{
    bool flag = false;
    int index = -1;
    for (int i = 0; i < s.size(); i++) {

        // Here I am replacing a character with "" and
        // then finding the length after replacing and
        // then subtracting  length of that replaced
        // string from the total length of the original
        // string
        string temp = s;
        temp.erase(remove(temp.begin(), temp.end(), s[i]), temp.end());  
        
        int count_occurrence = s.size() - temp.size();

        if (count_occurrence == 1)
        {
            flag = true;
            index = i;
            break;
        }
    }

    if (flag)
        cout << "First non repeating character is " << s[index] << endl;

    else
        cout << "There is no non-repeating character is present in the string" << endl;
}



int main(){
    
    // Driver Code
    string s = "GeeksforGeeks";
    firstNonRepeating(s);
    
    return 0;
}

// The code is contributed by Nidhi goel.