1289. 序列的第k个数

https://www.acwing.com/problem/content/submission/code_detail/13569668/

AcWing/1289/1289.cpp

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+#include <iostream>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int mod = 200907;
+
+int t, a, b, c, k;
+
+int binpow(int a, int b) {
+    int res = 1;
+    a %= mod;
+    while (b) {
+        if (b & 1) res = 1ll * res * a % mod;
+        a = 1ll * a * a % mod;
+        b >>= 1;
+    }
+    return res;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+
+    cin >> t;
+
+    while (t--) {
+        cin >> a >> b >> c >> k;
+
+        if (a - b == b - c) {  // 等差数列
+            cout << (a + 1ll * (b - a) * (k - 1)) % mod << endl;
+        } else {  // 等比数列
+            cout << 1ll * a * binpow(b / a, k - 1) % mod << endl;
+        }
+    }
+
+    return 0;
+}