2457. Minimum Addition to Make Integer Beautiful

You are given two positive integers n and target.

An integer is considered beautiful if the sum of its digits is less than or equal to target.

Return the minimum non-negative integer x such that n + x is beautiful. The input will be generated such that it is always possible to make n beautiful.

class Solution {
    long long find(long long n) {
        long long res = 0, p = 1;
        long long carry = 0;
        while(n) {
            long long now = carry + n % 10;
            carry = now / 10;
            now = now % 10;
            n /= 10;
            if(now) {
                res += (10 - now) * p;
                carry += 1;
            }
            p *= 10ll;
        }
        return res;
    }
    bool check(long long n, int target) {
        long long sum = 0;
        while(n) {
            sum += n % 10;
            n /= 10;
        }
        return sum <= target;
    }
    long long __stoll(string& s) {
        long long res = 0, p = 1;
        for(int i = s.length() - 1; i >= 0; i--) {
            res += (s[i] - '0') * p;
            p *= 10;
        }
        return res;
    }
public:
    long long makeIntegerBeautiful(long long n, int target) {
        long long res = find(n);
        string s = to_string(res);
        while(s.length()) {
            string ns = s.substr(1);
            long long now = __stoll(ns);
            if(check(now + n, target)) res = min(res,now);
            s = ns;
        }

        return res;
    }
};