统计一个数组中,每种值得个数
var cards = [1, 2, 3, 4, 3, 2, 1, 4, 5]
var dict = {};
for(var i = 0; i < cards.length; i ++){
dict[cards[i]] = dict[cards[i]] ? dict[cards[i]] + 1 : 1;
}
console.log(dict);
// { '1': 2, '2': 2, '3': 2, '4': 2, '5': 1 }