2283. Check if Number Has Equal Digit Count and Digit Value

You are given a 0-indexed string num of length n consisting of digits.

Return true if for every index i in the range 0 <= i < n, the digit i occurs num[i] times in num, otherwise return false.

class Solution {
public:
    bool digitCount(string num) {
        unordered_map<char, int> freq;
        for(auto& ch : num) freq[ch]++;
        for(int i = 0; i < num.length(); i++) {
            char ch = i + '0';
            int count = num[i] - '0';
            if(freq[ch] != count) return false;
        }
        return true;
    }
};