[剑指offer]左旋转字符串

思路：
怎么做也行，这里按照题目进行真的循环移动，通过建立一个char数组
实现：

public class Solution {
    public String LeftRotateString(String str,int n) {
        char[] chars = str.toCharArray();
        if(chars.length < n) return "";
        reverse(chars, 0, n-1);
        reverse(chars, n, chars.length-1);
        reverse(chars, 0, chars.length-1);
        StringBuilder sb = new StringBuilder(chars.length);
        for(char c:chars){
            sb.append(c);
        }
        return sb.toString();
    }
     
    public void reverse(char[] chars,int low,int high){
        char temp;
        while(low<high){
            temp = chars[low];
            chars[low] = chars[high];
            chars[high] = temp;
            low++;
            high--;
        }
    }
}