# Find First non-repeating character in a string

## In the given stream of characters(string), find the first non-repeating character.

### Example

Input string : ababcef

Output : c

Time complexity : O(1)

## Algorithm

Here we use doubly linked list, The DLL contains all non-repeating character in order.

1. Create an empty DLL, array inDLL[] and repeated[] of size of asci values.

2. inDLL[] contains pointers of DLL nodes.inDLL[x] has pointer of x, else NULL.

3. Repeated[] array has bool values, repeated[x] is true if x is repeated two or more times, else false.

4. Initialize inDLL[] with NULL values and repeated[x] is false.

5. The head of DLL is first non-repeating character.

6. For every new character x,

a) If repeated[x] is true, ignore this character.

b) If repeated[x] is false and inDLL[x] is Null Append x to DLL and store new DLL node in inDLL[x].

c) If repeadted[x] is false and inDLL[x] is not Null, get DLL node of x using inDLL[x] and remove the node. mark inDLL[x] as Null and repeated[x] as true.

## C++ Program

``````#include <bits/stdc++.h>

#define ASCII_VALUES 256
using namespace std;

struct DLLNode
{
char data;
struct DLLNode *next, *prev;
};
//function to insert node at beginning of DLL
void insertAtbeginning(struct DLLNode **head_ref, struct DLLNode **tail_ref,char x)
{
struct DLLNode *temp = new DLLNode;
temp->data = x;
temp->prev = temp->next = NULL;
{
return;
}
(*tail_ref)->next = temp;
temp->prev = *tail_ref;
*tail_ref = temp;
}

//Function to remove node
void DeleteNode(struct DLLNode **head_ref, struct DLLNode **tail_ref,struct DLLNode *temp)
{
{
return;
}
{
}
if(*tail_ref == temp)
{
*tail_ref = (*tail_ref)->prev;
}
if (temp->next != NULL)
{
temp->next->prev = temp->prev;
}
if (temp->prev != NULL)
{
temp->prev->next = temp->next;
}
delete(temp);
}

void FindFirst()
{
struct DLLNode *inDLL[ASCII_VALUES];
bool repeated[ASCII_VALUES];
// Initialize the above two arrays
struct DLLNode *head = NULL, *tail = NULL;
for (int i = 0; i < ASCII_VALUES; i++)
{
inDLL[i] = NULL;
repeated[i] = false;
}
//Traverse for all charcters
char string[] = "ababcef";
for (int i = 0; string[i]; i++)
{
char x = string[i];
cout<<"Reading charcter "<<x<<" from string \n";
//If charchter occurred less than twice.
if (!repeated[x])
{
if (inDLL[x] == NULL)//If it  is not in DLL add it in beginning
{
inDLL[x] = tail;
}
else//If its already there remove this character from DLL
{
inDLL[x] = NULL;
repeated[x] = true; // Also mark it as repeated
}
}
{
cout<<"First non-repeating character till now: "<<head->data<<endl;
}
}
}

//Main function
int main()
{
FindFirst();
return 0;
}``````

Scroll to Top