387. First Unique Character in a String

class Solution {
public:
    int firstUniqChar(string s) {
        unordered_map<char ,int> mp;
        for(int i = 0; i < s.size(); i++)
        {
            int count = 0;
            char str = s[i];
            size_t found = s.find(str);
            while(found != string::npos)
            {
                count++;
                found = s.find(str, found + 1);
                if(count > 1)
                {
                    break;
                }
            }
            mp[str] = count;
            // mp.insert({s[i], i});
            if(mp.at(s[i]) == 1)
            {
                return i;
            }
        }
        return -1;
    }
};
ExtrAA(SET vs MAP)
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Q) difference between unordered_set and unordered_map? why unordered_set cannot be used in this question?
ANS:
The primary difference between `unordered_set` and `unordered_map` in C++ lies in their structure and intended use:

### 1. `unordered_set`
- **Definition**: `unordered_set` is a collection of unique elements. Each element is stored only once, without any association to other values.
- **Usage**: Primarily used when you need to check for the presence or absence of elements (e.g., checking if an item is in a collection) without 
needing to store any additional information about each element.
- **Key Characteristics**:
  - Only stores **keys** (no associated values).
  - Does **not** allow duplicate entries.
  - Provides **O(1)** average-time complexity for insertion, deletion, and lookup.

### 2. `unordered_map`
- **Definition**: `unordered_map` is a collection of key-value pairs, where each key is associated with a specific value.
- **Usage**: Used when you need to store and access data based on a key, especially when you need to associate additional information (e.g., counts or indices) with each key.
- **Key Characteristics**:
  - Stores both **keys** and **values**.
  - Does **not** allow duplicate keys, but values associated with keys can be updated.
  - Provides **O(1)** average-time complexity for insertion, deletion, and lookup.

### Why `unordered_set` Cannot Be Used in This Question
In this question, you need to **track the frequency (count) of each character** in the string. `unordered_map` is suitable here because it allows you to store each character 
(as the key) along with its frequency (as the value). This makes it easy to look up how many times each character appears.

Using `unordered_set` would not work because:
- `unordered_set` only stores keys (characters), not values (frequency counts).
- You cannot associate a count with each character using `unordered_set`.

Therefore, `unordered_map` is necessary for problems that require storing extra information alongside each unique element.
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Easy Implementaion by ChatGPT:
int firstUniqChar(string s) {
    unordered_map<char, int> mp;

    for (char c : s) mp[c]++;
    for (int i = 0; i < s.size(); i++) {
        if (mp[s[i]] == 1) return i;
    }

    return -1;
}