LeetCode459. Repeated Substring Pattern

Given a non-empty string check if it can be constructed by taking a substring of it and appending multiple copies of the substring together. You may assume the given string consists of lowercase English letters only and its length will not exceed 10000.
Example 1:
Input: "abab"Output: TrueExplanation: It's the substring "ab" twice.
Example 2:
Input: "aba"Output: False
Example 3:
Input: "abcabcabcabc"Output: TrueExplanation: It's the substring "abc" four times. (And the substring "abcabc" twice.)

C++:

class Solution {
public:
    bool repeatedSubstringPattern(string str) {
        int len = str.size();
        for(int i=len/2;i>0;i--){
            if(len%i==0){
                int n = len/i;
                string sub = str.substr(0,i);
                string sb;
                for(int j=0;j<n;j++){
                    sb += sub;
                }
                if(sb==str) return true;
            }
        }
        return false;
    }
};

Java:

public class Solution {
    public boolean repeatedSubstringPattern(String str) {
        int len = str.length();
        for(int i=len/2;i>0;i--){
            if(len%i==0){
                int n = len/i;
                String sub = str.substring(0,i);
                StringBuffer sb = new StringBuffer();
                for(int j=0;j<n;j++){
                    sb.append(sub);
                }
                if(sb.toString().equals(str)) return true;
            }
        }
        return false;
    }
}