4. Рекурсия

План:
Триъгълни числа
Пермутации
Ефективност на рекурсията
Непряка (взаимна) рекурсия
** До сега за рекурсията
Записване на число с думи на английски език (7. Функции II)

** Триъгълни числа
// triangle.cpp
#include <iostream>
using namespace std
/**
   A class that describes triangle shapes like this:
   []
   [][]
   [][][]
   . . .
*/   
class Triangle {
public:
   Triangle(int w);
   int get_area() const;
private:
   int width;  
};
/**
   Constructs a triangle with a given width.
   @param w the width of the triangle base
*/
Triangle::Triangle(int w)
{  width = w;  }
/**
   Computes the area of the triangle shape.
   @return the area
*/
int Triangle::get_area() const
{  if (width == 1) return 1;
   Triangle smaller_triangle(width - 1);
   int smaller_area = smaller_triangle.get_area();
   return smaller_area + width;
}

int main()
{  Triangle t(4);
   cout << "Area: " << t.get_area() << endl;
   return 0;
}
**  Пермутации
// permute.cpp
#include <iostream>
#include <string>
#include <vector>
using namespace std;
/**
   Computes n!
   @param n a nonnegative integer
   @return n! = 1 * 2 * 3 * . . . * n
*/
int factorial(int n)
{  if (n == 0) return 1;
   int smaller_factorial = factorial(n - 1);
   int result = smaller_factoria * n;
   return result;
}
/**
   Generates all permutations of the characters in a string
   @param word a string
   @return a vector that is filled with all permutations 
   of the word
*/
vector<string> generate_permutations(string word)
{  vector<string> result;
   if (word.length() == 1) 
   {  result.push_back(word);
      return result;
   }

   for (int i = 0; i < word.length(); i++)
   {  string shorter_word = word.substr(0, i)
         + word.substr(i + 1, word.length() - i - 1);
      vector<string> shorter_permutations
         = generate_permutations(shorter_word);
      for (int j = 0; j < shorter_permutations.size(); j++)
      {  string longer_word = word[i] + shorter_permutations[j];
         result.push_back(longer_word);
      }   
   }
   return result;
}

int main()
{  cout << "Enter a string: ";
   string input;
   getline(cin, input);   
   cout << "There are " << factorial(input.length()) 
      << "permutations.\n";
   vector<string> v = generate_permutations(input);
   for (int i = 0; i < v.size(); i++)
      cout << v[i] << endl;
   return 0;
}

recursive dancing
** Ефективност на рекурсията
** Непряка (взаимна) рекурсия (mutual recurson) 

 // eval.cpp
 #include <iostream>
 using namespace std;

 int term_value();
 int factor_value();
 /**
    Evaluates the next expression found in cin
    @return the value of the expression.
 */
 int expression_value()
 {  int result = term_value();
    bool more = true;
    while (more)
    {  char op = cin.peek();
       if (op == '+' || op == '-')
       {  cin.get();
          int value = term_value();
          if (op == '+') result = result + value;
          else result = result - value;
       }
       else more = false;
    }
    return result;
 }
 /**
    Evaluates the next term found in cin
    @return the value of the term.
 */
 int term_value()
 {  int result = factor_value();
    bool more = true;
    while (more)
    {  char op = cin.peek();
       if (op == '*' || op == '/')
       {  cin.get();
          int value = factor_value();
          if (op == '*') result = result * value;
          else result = result / value;
       }
       else more = false;
    }
    return result;
 }
 /**
    Evaluates the next factor found in cin
    @return the value of the factor.
 */
 int factor_value()
 {  int result = 0;
    char c = cin.peek();
    if (c == '(')
    {  cin.get();
       result = expression_value();
       cin.get(); // read ")"
    }
    else // assemble number value from digits
    {  while (isdigit(c))
       {  result = 10 * result + c - '0';
          cin.get();
          c = cin.peek();
       } 
    }
    return result;
 }
 
 int main()
 {  cout << "Enter an expression: ";
    cout << expression_value() << endl;
    return 0;
 }