很简单：截取后再组合嘛

    public String LeftRotateString(String str,int n) {
        if(str.length()==0){
            return "";
        }
        StringBuilder sb = new StringBuilder(str);
        StringBuilder s1 = new StringBuilder(str.substring(0,n));
        StringBuilder s2 = new StringBuilder(str.substring(n,str.length()));
        String s = new String(s2.append(s1));
        return s;
    }