1247. Minimum Swaps to Make Strings Equal

class Solution {
public:
    int minimumSwap(string A, string B) {
        auto cnt = [&](auto target) -> int {
            return count_if(A.begin(), A.end(), [=](auto c){ return c == target; }) +
                   count_if(B.begin(), B.end(), [=](auto c){ return c == target; });
        };
        auto x = cnt('x'),
             y = cnt('y');
        if (x % 2 || y % 2)
            return -1;
        auto a = 0,
             b = 0;
        for (auto i=0; i < A.size(); ++i) {
            if (A[i] == B[i])
                continue;
            a += A[i] == 'x';
            b += B[i] == 'x';
        }
        return (a / 2) + (b / 2) + ((a % 2) + (b % 2));
    }
};

1248. Count Number of Nice Subarrays

Exactly K times = at most K times - at most K - 1 times

class Solution {
public:
    int numberOfSubarrays(vector<int>& nums, int k) {
       return atMost(nums, k) - atMost(nums, k-1);
    }
    
    int atMost(vector<int> &nums, int k){
        int res = 0;
        int i = 0;
        int n = nums.size();
        for(int j=0; j<n; j++){
            k -= nums[j] % 2;
            while(k < 0) k += nums[i++] % 2;
            res += j-i+1;
        }
        return res;
    }
};

Minimum Remove to Make Valid Parentheses

class Solution {
public:
    string minRemoveToMakeValid(string s) {
        stack<int> st;
        for(auto i = 0; i < s.size(); i++){
            if(s[i] == '(') st.push(i);
            if(s[i] == ')'){
                if(!st.empty()) st.pop();
                else s[i] = '*';
            }
        }
        while(!st.empty()){
            s[st.top()] = '*';
            st.pop();
        }
        s.erase(remove(s.begin(), s.end(), '*'), s.end());
        return s;
    }
};

1250. Check If It Is a Good Array

class Solution {
public:
    bool isGoodArray(vector<int>& A) {
        int res = A[0];
        for (int a: A)
            res = gcd(res, a);
        return res == 1;
    }
};