> For the complete documentation index, see [llms.txt](https://gl01.gitbook.io/gfg-editorials/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://gl01.gitbook.io/gfg-editorials/2023/07-2023-july-19/20-non-repeating-character.md).

# 20. Non Repeating Character

The problem can be found at the following link: [Question Link](https://practice.geeksforgeeks.org/problems/non-repeating-character-1587115620/1)

## My Approach

To find the first non-repeating character in a given string `S`, I use an array `cnt` of size 26 to count the occurrences of each character. I iterate through the string once to populate the `cnt` array. Then, I traverse the string again to find the first character whose count is equal to 1, indicating it is non-repeating.

## Time and Auxiliary Space Complexity

* **Time Complexity**: `O(N)`, where `N` is the length of the input string `S`.
* **Auxiliary Space Complexity**: `O(1)` (constant space) since the `cnt` array has a fixed size of `26`, which does not depend on the input.

## Code (C++)

```cpp
class Solution {
public:
    char nonrepeatingCharacter(string S) {
        int cnt[26] = {0};

        for (auto ch : S) {
            cnt[ch - 'a']++;
        }

        for (auto ch : S) {
            if (cnt[ch - 'a'] == 1) {
                return ch;
            }
        }

        return '$';
    }
};
```

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