console.log("for倒序");

for (i=arr.length-1; i>=0; i--){

    console.log("Hello, " + arr[i] + "!");

}

console.log("for正序");

for (var a in arr){

console.log(`Hello, ${arr[a]}!`);

}

console.log("while正序");

var l = arr.length;

var i = 0;

while(i < l){

console.log("Hello, " + arr[i] + "!");

i = i + 1;

}

console.log("while倒序")

var i = arr.length - 1;

while(i>=0){

console.log("Hello, " + arr[i] + "!");

i = i - 1;

}

console.log("do while正序");

var i = 0;

do{

console.log("Hello, " + arr[i] + "!");

i++;

} while (i < arr.length);

console.log("do while倒序");

var i = arr.length - 1;

do {

console.log("Hello, " + arr[i] + "!");

i--;

} while (i >= 0);