459. 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: True
Explanation: It's the substring "ab" twice.

Example 2:

Input: "aba"
Output: False

Example 3:

Input: "abcabcabcabc"
Output: True
Explanation: It's the substring "abc" four 

//Time : O(n), Space: O(1)    
public boolean repeatedSubstringPattern(String s) {
        if (s == null || s.length() == 0) {
            return true;
        }
        
        int len = s.length();
        
        for (int i = len / 2; i > 0; i--) {//注意：遍历到第二位停止，这样保证sub至少有一位
            if (len % i == 0) {//一定要在能整除的情况下做以下判断
                int count = len / i;
                StringBuilder sb = new StringBuilder();
                String sub = s.substring(0, i);
                
                for (int j = 0; j < count; j++) {
                    sb.append(sub);
                }
                
                if (sb.toString().equals(s)) {
                    return true;
                }
            }
        }
        
        return false;
    }