459. Repeated Substring Pattern

Problem:

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 times. (And the substring "abcabc" twice.)

Analysis:
The qualified substring must start from index 0. And its length <= s.length/2. So we find all candidate substrings and test whether they can fit s.

Solution:

class Solution {  
   public boolean repeatedSubstringPattern(String s) {  
           if (s == null || s.length() == 0)  
                return false;  
           int len = s.length();  
           for (int i = len/2; i >= 1; i--) {  
                if (len%i == 0) {  
                     StringBuilder sb = new StringBuilder();  
                     for (int j = 0; j < len/i; j++) {  
                          sb.append(s.substring(0, i));  
                     }  
                     if (s.equals(sb.toString()))   
                          return true;  
                }  
           }  
           return false;  
   }  
 }