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"

思路：

这道题是第i+1个字符串是第i个字符串的读法。比如第4个字符串是1211，它的读法是1个1，1个2,2个1，因此第5个字符串是111221

var countAndSay = function(n) {
    if(n<=0) return "";
    var res='1';
    for(var i=0;i<n-1;i++){
        var cur="";
        for(var j=0;j<res.length;++j){
            var count=1;
            while(j+1<res.length && res[j]==res[j+1]){
                ++count;
                ++j;
            }
            cur+=count+res[j]
        }
        res=cur;
    }
    return res;
};