LeetCode17.电话号码的字母组合 JavaScript

/**
 * @param {string} digits
 * @return {string[]}
 */
var letterCombinations = function(digits) {
  
    digits+="";
    var arr={2: [‘a‘, ‘b‘, ‘c‘],
            3: [‘d‘, ‘e‘, ‘f‘],
            4: [‘g‘, ‘h‘, ‘i‘],
            5: [‘j‘, ‘k‘, ‘l‘],
                6: [‘m‘, ‘n‘, ‘o‘],
            7: [‘p‘, ‘q‘, ‘r‘, ‘s‘],
            8: [‘t‘, ‘u‘, ‘v‘],
            9: [‘w‘, ‘x‘, ‘y‘, ‘z‘]
    };//先设一个数组
   var item=[];
   for(var i=0;i<digits.length;i++){
           var num=digits[i];
           if(item=="")
            item=arr[num];
           else{
               var list=[];
                   for(j=0;j<item.length;j++){
                       for(k=0;k<arr[num].length;k++){
                           list.push(item[j]+arr[num][k])
                       }
                   }
                   item=list
           }
   }
   return item;
};