JavaScript程序设计高级算法之动态规划实例分析

// 斐波那契数列
function recurFib(n) {
    if(n < 2){
      return n ;
    }else {
//          document.write("第"+(n-1)+"次计算&nbsp;n-1="+(n-1)+recurFib(n-1)+'&nbsp;&nbsp;&nbsp;');
//          document.write("n-2="+(n-2)+recurFib(n-2)+"<br>");
      return recurFib(n-1)+recurFib(n-2)
    }
}

function dynFib(n) {
  let val = [];
  for(let i = 0; i <= n; ++i){
    val[i]=0;
  }
  if(n ===1 || n === 2){
    return 1;
  }
  else {
    val[1] =1;
    val[2] = 2;
    for(let i = 3; i <= n; ++i){
      val[i] = val [i-1] +val[i-2] ;
    }
  }
  return val[n-1]
}

// 定义一个测试函数，将待测函数作为参数传入
function test(func,n){
  let start = new Date().getTime();//起始时间
  let res = func(n);//执行待测函数
  document.write('<br>'+'当n='+n+'的时候&nbsp;'+res+'<br>');
  let end = new Date().getTime();//结束时间
  return (end - start)+"ms";//返回函数执行需要时间
}

let time = test(recurFib,40);
document.write(time);
let time2 = test(dynFib,40);
document.write(time2);

function iterFib(n) {
  let last = 1;
  let nextLast = 1;
  let result = 1;
  for (let i = 2; i < n; ++i) {
    result = last + nextLast;
    nextLast = last;
    last = result;
  }
  return result;
}