Bear and Steady Gene

• Time : O(n)
• Space : O(1)

bool qualify(vector<int>& freq, vector<int>& A) {
    for(int i = 0; i < 4; i++) {
        if(A[i] and freq[i] < A[i]) return false;
    }
    return true;
}

int steadyGene(string gene) {
    vector<int> A(4, 0);
    auto mapping = [](char& ch) {
        if(ch == 'C') return 0;
        if(ch == 'G') return 1;
        if(ch == 'A') return 2;
        return 3;
    };
    for(auto& ch : gene) {
        A[mapping(ch)]++;
    }
    int n = gene.length(), stable = n / 4;
    for(auto& f : A) f = max(0, f - stable);
    if(!accumulate(begin(A), end(A), 0)) return 0;
    vector<int> freq(4, 0);
    int l = 0, r = 0, res = n;

    while(r < n) {
        while(r < n and !qualify(freq, A)) {
            freq[mapping(gene[r++])]++;
        }
        while(l < r and qualify(freq, A)) {
            res = min(res, r - l);
            freq[mapping(gene[l++])]--;
        }
    }
    return res;
}