循环左移字符串

public class Solution {
    public String LeftRotateString(String str,int n) {
        if(str==null  )
            return null;
        if(str.length()==0 || str.length()<n)
            return "";
        int len = str.length();
        n = n % len;
        str += str;
        return str.substring(n, n+len);
    }
}