StringBuffer(str.substring(0,n))截取0-（n-1）之间的字符串

import java.lang.StringBuffer;
public class Solution {
    public String LeftRotateString(String str,int n) {
        if(str.length()<n){
            return "";
        }
        if(str!=null){
        StringBuffer stf1=new StringBuffer(str.substring(0,n));
        StringBuffer stf2=new StringBuffer(str.substring(n,str.length()));
        stf1.reverse();
        stf2.reverse();
         return stf1.append(stf2).reverse().toString();
        }
        return "";
    }
}