# 17. First non-repeating character in a stream

The problem can be found at the following link: [Question Link](https://practice.geeksforgeeks.org/problems/first-non-repeating-character-in-a-stream1216/1)

## Approach

To find the first non-repeating character in a stream, follow these steps:

* Create a counter array, `cnt`, of size 26 to count the occurrences of characters in the input string. Initialize it with zeros.
* Initialize an empty vector called `last` to store the order of characters based on their first occurrence.
* Iterate through each character in the input string `A`.
  * Check if the current character is the first occurrence. If it is, add it to the `last` vector.
  * Find the first character in the `last` vector that has occurred only once.
  * If there are no characters with a single occurrence in the `last` vector, add '#' to the output string.
  * Otherwise, add the first occurrence element from the `last` vector to the output string.

## Time and Auxiliary Space Complexity

* **Time Complexity**: `O(n)`, where n is the length of the input string. We iterate through the string once.
* **Auxiliary Space Complexity**: `O(26)`, as the space used for `cnt` and `last` is constant, having a size of `26`.

## Code (C++)

```cpp
class Solution {
public:
    string FirstNonRepeating(string A) {
        vector<int> cnt(26, 0);
        vector<char> last;
        int firstOccur = 0;
        string out = "";

        for (int i = 0; i < A.size(); ++i) {
            cnt[A[i] - 'a']++;
            
            if (cnt[A[i] - 'a'] == 1)
                last.push_back(A[i]);
            
            while (firstOccur < last.size() && cnt[last[firstOccur] - 'a'] > 1) 
                ++firstOccur;
            
            if (firstOccur == last.size()) {
                out += '#';
            } else {
                out += last[firstOccur];
            }
        }
        return out;
    }
};
```

## Contribution and Support

For discussions, questions, or doubts related to this solution, please visit our [discussion section](https://github.com/getlost01/gfg-potd/discussions). We welcome your input and aim to foster a collaborative learning environment.

If you find this solution helpful, consider supporting us by giving a ⭐ star to the [getlost01/gfg-potd](https://github.com/getlost01/gfg-potd) repository.


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://gl01.gitbook.io/gfg-editorials/2023/07-2023-july-16/17-first-non-repeating-character-in-a-stream.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.