38. Count and Say（js）

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 where 1 ≤ n ≤ 30, 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"
题意：第一项是1个1，所以第二项是11，第二项是2个1，所以第三项是21，。。。。以此类推
代码如下：

/**
 * @param {number} n
 * @return {string}
 */
var countAndSay = function(n) {
    var s='1';
    for(var i=1;i<n;i++){
        s=getStr(s);
        // console.log(s)
    }
    // console.log(s);
    return s;
};
var getStr=function(s){

    var curr=s.charAt(0);
    var count=1;
    var res="";
    for(var i=1;i<s.length;i++){

        if(s.charAt(i)== curr){
            count++;
        }else{
            //与前一个字符不相同，立即将前面收集的字符存入res            
            res=res+count+curr;
            //当前字符存入curr
            curr=s.charAt(i);

            count=1;
        }
    }
    res=res+count+curr;
    return res+"";
};