support bazel complie this project and format code.

practical_exercises/10_day_practice/day2/compute.cpp

@@ -0,0 +1,26 @@
+/* 求π.cpp */
+#include <iostream>
+using namespace std;
+double arctan(double x);
+int main(int argc, char const *argv[]) {
+  double a = 16.0 * arctan(1.0 / 5.0);
+  double b = 4.0 * arctan(1.0 / 239.0);
+  double pi = a - b;
+  cout << pi << endl;
+  
+  return 0;
+}
+double arctan(double x) {
+  double head = x;
+  int tail = 1;
+  double art = 0;
+  while (double(head / tail) > 1e-15) {
+    art = (tail % 4 == 1) ? art + head / tail : art - head / tail;
+    head *= x * x;
+    tail += 2;
+    cout << "---------------" << endl;
+    cout << tail << endl;
+    cout << "---------------" << endl;
+  }
+  return art;
+}