常規(guī)循環(huán)：
function?sum(arr) {????
????var?s = 0;????
????for?(var?i=arr.length-1; i>=0; i--) {????????
????????s += arr[i];????
????}???
????return?s;
}
forEach遍歷：
function?sum(arr) {
????var?s = 0;
????arr.forEach(function(val, idx, arr) {
????????s += val;
????}, 0);
??????return?s;
};?
eval：
function?sum(arr) {
????return?eval(arr.join("+"));
};?
