Given a string, find its first non-repeating character

Given a string, find the first non-repeating character in it. For example, if the input string is “GeeksforGeeks”, then output should be ‘f’ and if input string is “GeeksQuiz”, then output should be ‘G’. We can use string characters as index and build a count array. Following is the algorithm.

1) Scan the string from left to right and construct the count array.
2) Again, scan the string from left to right and check for count of each
character, if you find an element who's count is 1, return it.

Example:

Input string: str = geeksforgeeks
1: Construct character count array from the input string.
....
count['e'] = 4
count['f'] = 1
count['g'] = 2
count['k'] = 2
……
2: Get the first character who's count is 1 ('f').

Implementation:

C/C++

 // C program to find first non-repeating character #include #include #define NO_OF_CHARS 256    /* Returns an array of size 256 containg count    of characters in the passed char array */ int *getCharCountArray(char *str) {    int *count = (int *)calloc(sizeof(int), NO_OF_CHARS);    int i;    for (i = 0; *(str+i);  i++)       count[*(str+i)]++;    return count; }    /* The function returns index of first non-repeating    character in a string. If all characters are repeating     then returns -1 */ int firstNonRepeating(char *str) {   int *count = getCharCountArray(str);   int index = -1, i;      for (i = 0; *(str+i);  i++)   {     if (count[*(str+i)] == 1)     {       index = i;       break;     }      }          free(count); // To avoid memory leak   return index; }    /* Driver program to test above function */ int main() {   char str[] = "geeksforgeeks";   int index =  firstNonRepeating(str);   if (index == -1)       printf("Either all characters are repeating or string is empty");   else    printf("First non-repeating character is %c", str[index]);   getchar();   return 0; }

Java

 // Java program to find first non-repeating character    class GFG  {     static final int NO_OF_CHARS = 256;     static char count[] = new char[NO_OF_CHARS];            /* calculate count of characters         in the passed string */      static void getCharCountArray(String str)       {          for (int i = 0; i < str.length();  i++)               count[str.charAt(i)]++;      }             /* The method returns index of first non-repeating        character in a string. If all characters are repeating         then returns -1 */     static int firstNonRepeating(String str)     {         getCharCountArray(str);         int index = -1, i;                 for (i = 0; i < str.length();  i++)         {             if (count[str.charAt(i)] == 1)             {                 index = i;                 break;             }            }                  return index;     }        // Driver method     public static void main (String[] args)     {         String str = "geeksforgeeks";         int index =  firstNonRepeating(str);                    System.out.println(index == -1 ? "Either all characters are repeating or string " +                    "is empty" : "First non-repeating character is " +  str.charAt(index));     } }

Python

 # Python program to print the first non-repeating character NO_OF_CHARS = 256    # Returns an array of size 256 containg count # of characters in the passed char array def getCharCountArray(string):     count =  * NO_OF_CHARS     for i in string:         count[ord(i)]+=1     return count    # The function returns index of first non-repeating # character in a string. If all characters are repeating # then returns -1 def firstNonRepeating(string):     count = getCharCountArray(string)     index = -1     k = 0        for i in string:         if count[ord(i)] == 1:             index = k             break         k += 1        return index    # Driver program to test above function string = "geeksforgeeks" index = firstNonRepeating(string) if index==1:     print "Either all characters are repeating or string is empty" else:     print "First non-repeating character is " + string[index]    # This code is contributed by Bhavya Jain

C#

 // C# program to find first non-repeating character using System; using System.Globalization;    class GFG {            static int NO_OF_CHARS = 256;     static char []count = new char[NO_OF_CHARS];            /* calculate count of characters      in the passed string */     static void getCharCountArray(string str)      {         for (int i = 0; i < str.Length; i++)             count[str[i]]++;     }            /* The method returns index of first non-repeating     character in a string. If all characters are      repeating then returns -1 */     static int firstNonRepeating(string str)     {         getCharCountArray(str);         int index = -1, i;                for (i = 0; i < str.Length; i++)         {             if (count[str[i]] == 1)             {                 index = i;                 break;             }          }                 return index;     }            // Driver code     public static void Main()     {         string str = "geeksforgeeks";         int index = firstNonRepeating(str);                    Console.WriteLine(index == -1 ? "Either " +         "all characters are repeating or string " +         "is empty" : "First non-repeating character"         + " is " + str[index]);     } }    // This code is contributed by Sam007

PHP



Output:

First non-repeating character is f

Can we do it by traversing the string only once?
The above approach takes O(n) time, but in practice it can be improved. The first part of the algorithm runs through the string to construct the count array (in O(n) time). This is reasonable. But the second part about running through the string again just to find the first non-repeater is not good in practice. In real situations, your string is expected to be much larger than your alphabet. Take DNA sequences for example: they could be millions of letters long with an alphabet of just 4 letters. What happens if the non-repeater is at the end of the string? Then we would have to scan for a long time (again).
We can augment the count array by storing not just counts but also the index of the first time you encountered the character e.g. (3, 26) for ‘a’ meaning that ‘a’ got counted 3 times and the first time it was seen is at position 26. So when it comes to finding the first non-repeater, we just have to scan the count array, instead of the string. Thanks to Ben for suggesting this approach.

Following is implementation of the extended approach that traverses the input string only once.

C++

 // CPP program to find first non-repeating // character #include 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    reurns INT_MAX */ int firstNonRepeating(char *str) {     pair arr[NO_OF_CHARS];        for (int i = 0; str[i]; i++)     {         (arr[str[i]].first)++;         arr[str[i]].second = i;     }        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].first == 1)             res = min(res, arr[i].second);        return res; }    /* Driver program to test above function */ int main() {     char str[] = "geeksforgeeks";     int index = firstNonRepeating(str);     if (index == INT_MAX)         printf("Either all characters are "               "repeating or string is empty");     else         printf("First non-repeating character"                " is %c", str[index]);     return 0; }

C

 #include #include #include #define NO_OF_CHARS 256    // Structure to store count of a character and index of the first // occurrence in the input string struct countIndex {    int count;    int index; };    /* Returns an array of above structure type. The size of    array is NO_OF_CHARS */ struct countIndex *getCharCountArray(char *str) {    struct countIndex *count =         (struct countIndex *)calloc(sizeof(countIndex), NO_OF_CHARS);    int i;    for (i = 0; *(str+i);  i++)    {       (count[*(str+i)].count)++;          // If it's first occurrence, then store the index       if (count[*(str+i)].count == 1)          count[*(str+i)].index = i;    }    return count; }    /* The function returns index of the first non-repeating     character in a string. If all characters are repeating     then reurns INT_MAX */ int firstNonRepeating(char *str) {   struct countIndex *count = getCharCountArray(str);   int result = INT_MAX, i;      for (i = 0; i < NO_OF_CHARS;  i++)   {     // If this character occurs only once and appears     // before the current result, then update the result     if (count[i].count == 1 && result > count[i].index)        result = count[i].index;   }      free(count); // To avoid memory leak   return result; }    /* Driver program to test above function */ int main() {   char str[] = "geeksforgeeks";   int index =  firstNonRepeating(str);   if (index == INT_MAX)     printf("Either all characters are repeating or string is empty");   else    printf("First non-repeating character is %c", str[index]);   getchar();   return 0; }

Java

 // Java program to find first non-repeating character // Note : hashmap is used    import java.util.HashMap;    class CountIndex {     int count,index;            // constructor for first occurrence     public CountIndex(int index) {         this.count = 1;         this.index = index;     }            // method for updating count     public void incCount() {         this.count++;     } } class GFG  {     static final int NO_OF_CHARS = 256;            static HashMap hm = new HashMap(NO_OF_CHARS);            /* calculate count of characters         in the passed string */      static void getCharCountArray(String str)       {          for (int i = 0; i < str.length();  i++)          {             // If character already occurred,               if(hm.containsKey(str.charAt(i)))              {                  // updating count                  hm.get(str.charAt(i)).incCount();              }                             // If it's first occurrence, then store the index and count = 1              else              {                  hm.put(str.charAt(i), new CountIndex(i));              }                              }          }             /* The method returns index of first non-repeating        character in a string. If all characters are repeating         then returns -1 */     static int firstNonRepeating(String str)     {         getCharCountArray(str);         int result = Integer.MAX_VALUE, i;                 for (i = 0; i < str.length();  i++)         {              // If this character occurs only once and appears             // before the current result, then update the result             if (hm.get(str.charAt(i)).count == 1 && result > hm.get(str.charAt(i)).index){                  result = hm.get(str.charAt(i)).index;             }                          }                  return result;     }        // Driver method     public static void main (String[] args)     {         String str = "geeksforgeeks";         int index =  firstNonRepeating(str);                    System.out.println(index == Integer.MAX_VALUE ? "Either all characters are repeating " +               " or string is empty" : "First non-repeating character is " +  str.charAt(index));     } }

C#

 // C# program to find first  // non-repeating character  // Note : hashmap is used  using System; using System.Collections.Generic;    class CountIndex {     public int count, index;        // constructor for first occurrence      public CountIndex(int index)     {         this.count = 1;         this.index = index;     }        // method for updating count      public virtual void incCount()     {         this.count++;     } }    class GFG {     public const int NO_OF_CHARS = 256;        public static Dictionary hm = new Dictionary(NO_OF_CHARS);        /* calculate count of characters      in the passed string */     public static void getCharCountArray(string str)     {         for (int i = 0; i < str.Length; i++)         {             // If character already occurred,              if (hm.ContainsKey(str[i]))             {                 // updating count                  hm[str[i]].incCount();             }                // If it's first occurrence, then              // store the index and count = 1              else             {                 hm[str[i]] = new CountIndex(i);             }            }     }        /* The method returns index of first      non-repeating character in a string.      If all characters are repeating then     returns -1 */     internal static int firstNonRepeating(string str)     {         getCharCountArray(str);         int result = int.MaxValue, i;            for (i = 0; i < str.Length; i++)         {             // If this character occurs only              // once and appears before the              // current result, then update the result              if (hm[str[i]].count == 1 &&                  result > hm[str[i]].index)             {                 result = hm[str[i]].index;             }            }        return result;     }        // Driver Code     public static void Main(string[] args)     {         string str = "geeksforgeeks";         int index = firstNonRepeating(str);            Console.WriteLine(index == int.MaxValue ?                            "Either all characters are repeating " +                            " or string is empty" :                            "First non-repeating character is " +                                                     str[index]);     } }    // This code is contributed by Shrikant13

Output:

First non-repeating character is f

Related Problem : K’th Non-repeating Character