LeetCode_459. Repeated Substring Pattern

459. Repeated Substring Pattern

Easy

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.)

package leetcode.easy;

public class RepeatedSubstringPattern {
	public boolean repeatedSubstringPattern(String s) {
		// corner case
		if (s == null || s.length() == 0) {
			return true;
		}
		int n = s.length();
		for (int len = n / 2; len >= 1; len--) {
			if (n % len != 0) {
				continue; // s length must can be divided by the pattern length
			} else {
				String pattern = s.substring(0, len); // pattern string
				int i = len; // start index of 2nd pattern
				int j = i + len - 1; // end index of 2nd pattern
				while (j < n) {
					String substr = s.substring(i, j + 1);
					if (!pattern.equals(substr)) {
						break; // failed for this pattern, try next pattern
					} else {
						i += len;
						j += len;
					}
				}
				// if it past the last substring check, i will be n
				if (i == n) {
					return true;
				}
			}
		}
		return false;
	}

	@org.junit.Test
	public void test() {
		System.out.println(repeatedSubstringPattern("abab"));
		System.out.println(repeatedSubstringPattern("aba"));
		System.out.println(repeatedSubstringPattern("abcabcabcabc"));
	}
}