#include <iostream>
using namespace std;

// 高精度 + 高精度
string add(string s1, string s2) {
  string r; // 存放最终的求和结果
  int a[1000] = {0};
  int b[1000] = {0};
  int c[1000] = {0};
  
  int len = max(s1.size(), s2.size());
  
  for (int i = s1.size() - 1, j = 0; i >= 0; i--, j++) {
    a[j] =s1[i] - '0';
  }
  
  for (int i = s2.size() - 1, j = 0; i >= 0; i--, j++) {
    b[j] =s2[i] - '0';
  }
  
  int t = 0; // 代表加法进位
  for (int i = 0; i < len; i++) {
    t = a[i] + b[i] + t;
    c[i] = t % 10;
    t = t / 10;
  }
  
  if (t > 0) c[len++] = t;
  
  for (int i = 0; i < len; i++) {
    r = char(c[i] + '0') + r;
  }
  
  return r;
}

// 高精度 × 2
string mul(string s) {
  string r;
  int a[1000] = {0};
  int c[1000] = {0};
  int len = s.size();
  
  for (int i = len - 1, j = 0; i >= 0; i--, j++) {
    a[j] = s[i] - '0';
  }
  
  int t = 0;
  for (int i = 0; i < len; i++) {
    t = t + a[i] * 2;
    c[i] = t % 10;
    t = t / 10;
  }
  
  if (t > 0) c[len++] = t;
  
  for (int i = 0; i < len; i++) {
    r = char(c[i] + '0') + r;
  }
  
  return r;
}

int main() {
  
  string x, y, z;
  int n;
  cin >> n;
  
  // 递推公式 A(n) = A(n-1) * 2 + A(n-2)
  x = "1";
  y = "2";
  
  if (n == 1) cout << x << endl;
  else if (n == 2) cout << y << endl;
  else {
    for (int i = 3; i <= n; i++) {
      z = add(mul(y), x);
      x = y;
      y = z;
    }
  }
  
  cout << z << endl;
  return 0;
}
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C++ Online Compiler

Write, Run & Share C++ code online using OneCompiler's C++ online compiler for free. It's one of the robust, feature-rich online compilers for C++ language, running on the latest version 17. Getting started with the OneCompiler's C++ compiler is simple and pretty fast. The editor shows sample boilerplate code when you choose language as C++ and start coding!

Read inputs from stdin

OneCompiler's C++ online compiler supports stdin and users can give inputs to programs using the STDIN textbox under the I/O tab. Following is a sample program which takes name as input and print your name with hello.

#include <iostream>
#include <string>
using namespace std;

int main() 
{
    string name;
    cout << "Enter name:";
    getline (cin, name);
    cout << "Hello " << name;
    return 0;
}

About C++

C++ is a widely used middle-level programming language.

  • Supports different platforms like Windows, various Linux flavours, MacOS etc
  • C++ supports OOPS concepts like Inheritance, Polymorphism, Encapsulation and Abstraction.
  • Case-sensitive
  • C++ is a compiler based language
  • C++ supports structured programming language
  • C++ provides alot of inbuilt functions and also supports dynamic memory allocation.
  • Like C, C++ also allows you to play with memory using Pointers.

Syntax help

Loops

1. If-Else:

When ever you want to perform a set of operations based on a condition If-Else is used.

if(conditional-expression) {
   //code
}
else {
   //code
}

You can also use if-else for nested Ifs and If-Else-If ladder when multiple conditions are to be performed on a single variable.

2. Switch:

Switch is an alternative to If-Else-If ladder.

switch(conditional-expression){    
case value1:    
 // code    
 break;  // optional  
case value2:    
 // code    
 break;  // optional  
......    
    
default:     
 code to be executed when all the above cases are not matched;    
}

3. For:

For loop is used to iterate a set of statements based on a condition.

for(Initialization; Condition; Increment/decrement){  
  //code  
}

4. While:

While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.

while (condition) {  
// code 
}

5. Do-While:

Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.

do {  
 // code 
} while (condition);

Functions

Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity. Function gets run only when it is called.

How to declare a Function:

return_type function_name(parameters);

How to call a Function:

function_name (parameters)

How to define a Function:

return_type function_name(parameters) {  
 // code
}