斐波那切数列

通过下标求值

• 生成数组

function fib(index){
   let arr = [1,1]
   for(let i = 2;i < index + 1;i++){
   　　arr.push(arr[i - 1] + arr[i - 2])
   }
   return arr[index]
} 

• 未生成数组

function fib(index){
    let pre = 1,cur = 1,next;
    if(index < 2){
        return 1
    }
    for(let i = 2;i < index + 1;i++){
        next = pre + cur
        pre = cur;
        cur = next;
    }
    return next
}

 非尾递归

function fib(index) {
    if (index <= 1){
        return 1
    };
    return fib(index - 1) + fib(index - 2);
}

尾递归

function fib(index,pre = 1,cur = 1){
    if(index <= 1){
        return cur
    }
    return fib(index - 1,cur,pre + cur)
}

通过值求下标

function fib(val){
    if(val == 1){
        return 0
    }
    let pre = 1,cur = 1,next;
    for(let i = 2;i < val;i++){
        next = pre + cur
        pre = cur;
        cur = next;
        if(val == next){
            return i
        }else if(next > val){
            return -1
        }
    }
}

function fib(val){
    if(val == 1){
        return 0
    }
    let pre = 1,cur = 1,next = pre + cur,i = 2;
    while(next <= val){
        if(next == val){
            return i
        }
        pre = cur;
        cur = next;
        next = pre + cur
        i++
    }
    return -1
}