题解

#include <iostream>
using namespace std;

int match[10] = {6, 2, 5, 5, 4, 5, 6, 3, 7, 6};
int numMatches[2000];  // 预处理0-1999的数字所需火柴数

int main() {
    int n;
    cin >> n;
    
    if (n < 13) {
        cout << 0 << endl;
        return 0;
    }
    
    // 预处理所有数字的火柴数
    for (int i = 0; i < 2000; i++) {
        int num = i;
        if (num == 0) {
            numMatches[i] = match[0];
            continue;
        }
        int total = 0;
        while (num > 0) {
            total += match[num % 10];
            num /= 10;
        }
        numMatches[i] = total;
    }
    
    int available = n - 4;
    int result = 0;
    
    // 枚举A和B
    for (int a = 0; a <= 999; a++) {
        for (int b = 0; b <= 999; b++) {
            int c = a + b;
            if (c >= 2000) continue;
            if (numMatches[a] + numMatches[b] + numMatches[c] == available) {
                result++;
            }
        }
    }
    
    cout << result << endl;
    return 0;
}