1 暴力遍历

取模，再大都不怕

public String LeftRotateString(String str,int n) {
    if(str==null || str.length() ==0){
        return null;
    }
    char [] chars=str.toCharArray();
    int len=chars.length
    char [] resChars=new char[len];
    for(int i=0;i<len;i++){
        int j=i-k%len
        if(j<0){
            j=len-j;
        }  
        resChars[j]=chars[i];
    }
    StringBuilder sb=new StringBuilder();
    for(char c : resChars){
        sb.append(c);
    }
    return sb.toString();
}

2 根据特带你的优化

public String LeftRotateString(String str,int n) {
    if(str==null || str.length() ==0){
        return "";
    }
    int len=str.length();
    int tmp=n%len;
    String str1=str.substring(0,tmp);
    String str2=str.substring(tmp,len);
    return str2+str1;
}