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无限区域

《模拟》

c++
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#include <iostream>
#include <cstring>
#include <algorithm>
#include <cmath>

using namespace std;

signed main()
{
    ios::sync_with_stdio(false),cin.tie(0),cout.tie(0);
    
    int t;
    cin >> t;
    double pi = acos(-1);
    for (int i = 1; i <= t; i ++)
    {
        int r, a, b;
        cin >> r >> a >> b;
        double ans = 0;
        while (r)
        {
            ans += pi * r * r;
            r *= a;
            ans += pi * r * r;
            r /= b;
        }
        printf("Case #%d: %.12f\n", i, ans);
    }
}

交替数字和

《模拟》

c++
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class Solution {
public:
    int alternateDigitSum(int n) {
        int ans = 0;
        string str = to_string(n);
        int pre = 1;
        for (auto ch : str)
        {
            ans += pre * (ch - '0');
            pre *= -1;
        }

        return ans;
    }
};