[Project Euler] Problem 2

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

  
    

   
     #include 
   
     <
   
     iostream
   
     >
   
     

   
     using
   
      
   
     namespace
   
      std;


   
     int
   
      main(){
 
   
     int
   
      sum 
   
     =
   
      
   
     0
   
     ;
 
   
     int
   
      a 
   
     =
   
      
   
     1
   
     , b 
   
     =
   
      
   
     2
   
     , c 
   
     =
   
      
   
     3
   
     ;
 
   
     while
   
     (b 
   
     <
   
      
   
     4000000
   
     ){
 sum 
   
     +=
   
      b;
 a 
   
     =
   
      b
   
     +
   
     c;
 b 
   
     =
   
      a
   
     +
   
     c;
 c 
   
     =
   
      a
   
     +
   
     b;
 }
 cout 
   
     <<
   
      sum 
   
     <<
   
      endl;
 
   
     return
   
      
   
     0
   
     ;
}
  
    

求所有Fibonacc数列中值为偶数的数的和

观察数列，容易看出每隔两个数有一个偶数

所以，我们每三个数递推一次