var length = arr.length

console.log('====正序：for====')

for (var i in arr){

    console.log('Hello, '+ arr[i]);

}

console.log('====倒序：for====')

for (i=1; i<=length; i++){

    console.log('Hello, '+arr[length-i])

}

console.log('====正序：while====')

i = 0

while (i < length){

    console.log('Hello, '+ arr[i])

    i ++

}

console.log('====倒序：while====')

i = 1

while (i <= length){

    console.log('Hello, '+ arr[length-i])

    i ++

}