Merge pull request #153 from AlgorithmWithGod/khj20006

[20250220] BOJ / G5 / 라그랑주의 네 제곱수 정리 / 권혁준

Author: ShinHeeEul

khj20006/202502/20 BOJ G5 라그랑주의 네 제곱수 정리.md

@@ -0,0 +1,40 @@
+```cpp
+
+#include <iostream>
+#include <algorithm>
+#include <vector>
+using namespace std;
+
+int main()
+{
+	cin.tie(0)->sync_with_stdio(0);
+
+	int N;
+	for (cin >> N; N; cin >> N) {
+
+		vector<int> A;
+		int ans = 0;
+		for (int i = 1; i*i <= N; i++) A.push_back(i*i);
+		int M = A.size();
+
+		// 1
+		if (A.back() == N) ans++;
+		// 2
+		for (int i = 0; i < M; i++) for (int j = i; j < M; j++) if (A[i] + A[j] == N) ans++;
+		// 3
+		for (int i = 0; i < M; i++) for (int j = i; j < M; j++) {
+			if (A[i] + A[j] >= N) break;
+			ans += upper_bound(A.begin() + j, A.end(), N - A[i] - A[j]) - lower_bound(A.begin() + j, A.end(), N - A[i] - A[j]);
+		}
+		// 4
+		for (int i = 0; i < M; i++) for (int j = i; j < M; j++) for (int k = j; k < M; k++) {
+			if (A[i] + A[j] + A[k] >= N) break;
+			ans += upper_bound(A.begin() + k, A.end(), N - A[i] - A[j] - A[k]) - lower_bound(A.begin() + k, A.end(), N - A[i] - A[j] - A[k]);
+		}
+
+		cout << ans << '\n';
+	}
+	
+}
+
+```