Tail Recursion Fibonacci

Question

How do I implement a recursive Fibonacci function with no loops running in O(n)?

Answer 1

In case someone is looking for a JavaScript solution:

function _fib(n, left, right) {
  switch (n) {
    case 0: return 0
    case 1: return right
    default: return _fib(n - 1, right, left + right)
  }
}

function fib(n) {
  return _fib(n, 0, 1)
}

That runs in O(n) time and O(1) space with tail call optimization.