火柴棒等式

https://www.luogu.com.cn/problem/P1149

先把所有的数表示出来

#include <cstdio>
#include <iostream>

using namespace std;
int b[1000] = {6, 2, 5, 5, 4, 5, 6, 3, 7, 6}, n, cnt, i;

int main() {
    cin >> n, n -= 4;

    for (i = 10; i <= 999; ++i)
        b[i] = b[i / 10] + b[i % 10];

    for (i = 0; i <= 999; ++i) {
        for (int j = 0; j <= i; ++j)
            if (b[i] + b[j] + b[i - j] == n)
                cnt++;
    }
    cout << cnt;
    return 0;
}