added day2 q3 solution

Author: MANISH-BEESA

Problems/Mathematics/Day-02/BEESA_MANISH/Solution3,cpp

@@ -0,0 +1,50 @@
+/*
+TC - O(N log N)
+SC - O(N)
+
+*/
+
+
+#include <iostream>
+#include <vector>
+
+using namespace std;
+
+long long MOD = 1000000007;
+const int MAXN = 1000005;
+int phi[MAXN];
+long long f[MAXN];
+
+void sieve(int n) {
+    for (int i = 0; i <= n; i++) phi[i] = i;
+    for (int i = 2; i <= n; i++) {
+        if (phi[i] == i) {
+            for (int j = i; j <= n; j += i)
+                phi[j] -= phi[j] / i;
+        }
+    }
+    for (int d = 1; d <= n; d++) {
+        for (int j = d; j <= n; j += d) {
+            f[j] = (f[j] + 1LL * d * phi[j / d]) % MOD;
+        }
+    }
+}
+
+int main() {
+    int N;
+    cin >> N;
+    sieve(N);
+
+    long long total_sum = 0;
+    for (int d = 1; d <= N; d++) {
+        long long current_f_sum = 0;
+        for (int k = d; k <= N; k += d) {
+            current_f_sum = (current_f_sum + f[k]) % MOD;
+        }
+        long long term = (1LL * phi[d] * ((current_f_sum * current_f_sum) % MOD)) % MOD;
+        total_sum = (total_sum + term) % MOD;
+    }
+
+    cout << total_sum << endl;
+    return 0;
+}