在scala中如果一个函数在最后一步调用自己（必须完全调用自己，不能加其他额外运算子），那么在scala中会复用函数栈，这样递归调用就转化成了线性的调用，效率大大的提高。
If a function calls itself as its last action, the function‘s stack frame can be reused. This is called tail recursion.
=> Tail recursive functions are iterative process
这里实现了两个版本的阶乘函数，一个是普通的写法，另外一个使用了tail recursive的优化
/* In Scala, only directly recursive call to the current function are optimized. One can require that a function is tail-recursive using a @tailrec annotation: @tailrec def gcd(a: Int, b: Int): Int = ... If the annotation is given, and the implementation of gcd were not tail recursive, an error would be issued. */ import scala.annotation.tailrec object exercise { def factorial(n: Int): Int = if (n == 0) 1 else n * factorial(n-1) //a tail recursive version of factorial def factorialTailRecursion(n: Int): Int = { @tailrec def loop(acc: Int, n: Int): Int = if (n == 0) acc else loop(acc * n, n-1) loop(1, n) } factorial(4) factorialTailRecursion(4) //optimized! the function‘s stack frame can be reused! }
原文：http://www.cnblogs.com/yanghuahui/p/4033563.html