[PAT-A] 1049 Counting Ones

Java

import java.util.*;

public class Main {
    public static int calc(int n) {
        List<Integer> nums = new ArrayList<>();
        while (n > 0) {
            nums.add(n % 10);
            n /= 10;
        }

        int res = 0;
        for (int i = nums.size() - 1; i >= 0; i--) {
            int d = nums.get(i);
            int left = 0, right = 0, power = 1;
            for (int j = nums.size() - 1; j > i; j--) left = left * 10 + nums.get(j);
            for (int j = i - 1; j >= 0; j--) {
                right = right * 10 + nums.get(j);
                power *= 10;
            }

            if (d == 0) res += left * power;
            else if (d == 1) res += left * power + right + 1;
            else res += (left + 1) * power;
        }

        return res;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();

        System.out.println(calc(n));
        scanner.close();
    }
}

C++

#include <iostream>
#include <cstring>
#include <algorithm>
#include <vector>

using namespace std;

int calc(int n) {
    vector<int> nums;
    while (n) {
        nums.push_back(n % 10);
        n /= 10;
    }

    int res = 0;
    for (int i = nums.size() - 1; i >= 0; i--) {
        int d = nums[i];
        int left = 0, right = 0, power = 1;
        for (int j = nums.size() - 1; j > i; j--) left = left * 10 + nums[j];
        for (int j = i - 1; j >= 0; j--) {
            right = right * 10 + nums[j];
            power *= 10;
        }

        if (d == 0) res += left * power;
        else if (d == 1) res += left * power + right + 1;
        else res += (left + 1) * power;
    }

    return res;
}

int main() {
    int n;
    cin >> n;

    cout << calc(n) << endl;

    return 0;
}