17. Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

这是回溯问题(参考：http://blog.csdn.net/jarvischu/article/details/16067319)。

下面是JavaScript代码编写的递归版和迭代版的代码：

递归版：
var letterCombinations = function(digits) {
    var map = ['','','abc','def','ghi','jkl','mno','pqrs','tuv','wxyz'];
    var result = [];
    backtrack(map, "", digits, result, 0);
    return result;
};
function backtrack(map, s, digits, result, start){
        
    if(digits.length === 0) return;
  
    if(s.length === digits.length){
         result.push(s);
         return;
    }
    else{
        for(var i = start; i < digits.length; i++){
        　　var index = digits[i] - '0';
           for(var j=0; j < map[index].length; j++){
           　　s = s + map[index][j];
           　　backtrack(map, s, digits, result, i+1);
           　　s = s.substring(0, s.length-1);
           }
         } 
     }  
}