左旋转字符串

class Solution {
    public String leftRotateString(String str,int n) {
        if (n == 0)
            return str;
        int len = str.length();
        StringBuilder sb = new StringBuilder();
        for (int i = n; i < len; i++) {
            sb.append(str.charAt(i));
        }
        for (int i = 0; i < n; i++) {
            sb.append(str.charAt(i));
        }
        return sb.toString();
    }
}