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

Solution1：intuitive

思路：
Time Complexity: O(N^2) Space Complexity: O(1)

Solution2：KMP

思路：
https://discuss.leetcode.com/topic/67590/java-o-n
http://www.cnblogs.com/grandyang/p/6087347.html
Time Complexity: O(N) Space Complexity: O(1)

Solution Code：

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

}