584C - Marina and Vasya - CodeForces Solution

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int main() {
    int n , t;
    cin>> n >>t;
    string a , b;
    cin>>a>>b;
    vector<int> index;
    unordered_map<int , bool> mp;
    for(int i = 0; i < n; i++) {
        if(a[i] == b[i]) index.push_back(i) , mp[i] = true;
    }
    t = n - t;
    int m = index.size();
    int tt= t - m;
    if((n - m) / 2 < tt) cout<<-1;
    else{
        string ans = a;
        if(m >= t) {
            for(int i = t; i < m; i++) {
                ans[index[i]] = 'a' + (ans[index[i]] - 'a' + 1) % 26;
            }
            for(int i = 0; i < n; i++) {
                if(mp.count(i)) continue;
                for(char x = 'a'; x <= 'z'; x++) {
                    if(x != a[i] && x != b[i]) {
                        ans[i] = x;
                        break;
                    }
                }
            }
        }else{
            int cnt1 = t - m;
            int cnt2 = cnt1;
            for(int i = 0; i < n; i++) {
                if(mp.count(i)) continue;
                if(cnt1 > 0){
                    ans[i] = b[i];
                    cnt1--;
                }else if(cnt2 > 0) {
                    ans[i] = a[i];
                    cnt2--;
                }else{
                    for(char x = 'a'; x <= 'z'; x++) {
                        if(x != a[i] && x != b[i]) {
                            ans[i] = x;
                            break;
                        }
                    }
                }
            }
        }
        cout<<ans;
    }
    system("pause");
    return 0;
}