左旋转字符串

public class Solution {
public String LeftRotateString(String str,int n) {
if(str==null||str.length()<n){
return str;
}
StringBuilder sb = new StringBuilder();
//左移几位，就抽取一个长度为几的字符串
String substring = str.substring(0, n);
String substring1 = str.substring(n, str.length());
//将字符串进行拼接，因为是左移，就把后面的字符串放到前面，将要移到的字符串放后面
sb.append(substring1);
sb.append(substring);
return sb.toString();
}
}