//倒序
var max = arr.length;
var i=0;
while(i<max){
    max --;
    alert("hello,"+arr[max]+"!");
}

for(var i=0;i<max;max--){
    alert("hello,"+arr[max-1]+"!");
}


//正序
var i=0;
while(i<max){
    alert("hello,"+arr[i]+"!");
    i++;
}


for(var i=0;i<max;i++){
    alert("hello,"+arr[i]+"!");
}