### Recursion tree diagram
```
fib(6)

 fib(5)
    fib(4)
       fib(3)
          fib(2)
             fib(1) = 1
             fib(0) = 0
         = 1 + 0 = 1
          fib(1) = 1
      = 1 + 1 = 2
       fib(2)
           fib(1) = 1
           fib(0) = 0
      = 1 + 0 = 1
   = 2 + 1 = 3
    fib(3)
        fib(2)
           fib(1) = 1
           fib(0) = 0
       = 1 + 0 = 1
        fib(1) = 1
   = 1 + 1 = 2
= 3 + 2 = 5
 fib(4)
     fib(3)
        fib(2)
           fib(1) = 1
           fib(0) = 0
       = 1 + 0 = 1
        fib(1) = 1
    = 1 + 1 = 2
     fib(2)
         fib(1) = 1
         fib(0) = 0
    = 1 + 0 = 1
= 2 + 1 = 3

fib(6) = fib(5) + fib(4) = 5 + 3 = 8
```