递归的优化方案

实现斐波那契数列:

• fib(1) = 1
• fib(2) = 1
• fib(n) = fib(n-1) + fib(n-2)

递归实现

function fib(n) {
  if(n === 1 || n === 2) return 1
  return fib(n-1) + fib(n-2)
}

非递归实现

function fn(n) {
  let last1 = 1, last2 = 1, temp
  for(let i = 3; i <= n; i++) {
    temp = last1 + last2
    last1 = last2
    last2 = temp
  }
  return last2
}

用空间换效率

function fib(n) {
  let arr = [0, 1, 1]
  let(i=3; i<=n; i++) {
    arr[i] = arr[i-1] + arr[i-2]
  }
  return arr[n]
}

使用缓存提升效率

function fib(n) {
  let cache = [0, 1, 1]
  function _fib(n) {
    if(cache[n]) return cache[n]
    cache[n] = _fib(n-1) + _fib(n-2)
    return cache[n]
  }
  return _fib(n)
}