leetcode练习：38. Count and Say

The count-and-say sequence is the sequence of integers with the first five terms as following:

1.     1
2.     11
3.     21
4.     1211
5.     111221

1 is read off as "one 1" or 11.
11 is read off as "two 1s" or 21.
21 is read off as "one 2, then one 1" or 1211.

Given an integer n, generate the nth term of the count-and-say sequence.

Note: Each term of the sequence of integers will be represented as a string.

Example 1:

Input: 1
Output: "1"

Example 2:

Input: 4
Output: "1211"

这个题目，一开始还真的没看懂~后来上网翻译了一下发现意思就是，数有几个1，几个2，这样，后面一个数前面一个，所以很明显就可以用递归做，当然所有的递归都可以用迭代写出来咯。

var countAndSay = function(n) {
    if(n==1)
        return "1";
    
    var last = countAndSay(n-1);
    
    var len = last.length;
    var num = 1;
    var res = "";
    
    for(var i=1;i<=len;i++){
        if(last[i] == last[i-1]){
            num++;
        } else {
           res = res + num + last[i-1];  
           num = 1;  
        }
    }
    
    return res;

};