暴力三重循环

#include <bits/stdc++.h>
using namespace std;
int main()
{
    int n;
    cin >> n;
    int ans = 0;
    for (int a = 1; a <= n; a++)
        for (int b = a; b <= n; b++)
            for (int c = 1; c <= n; c++)
                if (a * a + b * b == c * c)
                    ans++;
    cout << ans;
    return 0;
}

第三重循环直接生成+检验

#include <bits/stdc++.h>
using namespace std;
int main()
{
    int n;
    cin >> n;
    int ans = 0;
    for (int a = 1; a <= n; a++)
        for (int b = a; b <= n; b++)
        {
            int c = sqrt(a * a + b * b);
            if (c <= n && a * a + b * b == c * c)
                ans++;
        }
    cout << ans;
    return 0;
}