459. Repeated Substring Pattern

• Difficulty: Easy

• Topics: String

• Similar Questions:

  • Implement strStr()

  • Repeated String Match

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.)

Solutions:

class Solution {
public:
    bool repeatedSubstringPattern(string s) {
        int len = s.length();
        for (int i = 2; i <= len; ++i) {
            if (len % i == 0 && isPrime(i)) {
                int patternLen = len / i;
                int j = 0;
                for (j = 0; j < patternLen; ++j) {
                    int k = 1;
                    for (k = 1; k < i; ++k) {
                        if (s[k * patternLen + j] != s[j]) break;
                    }
                    if (k != i) break;
                }
                if (j == patternLen)    return true;
            }
        }
        return false;
    }

private:
    bool isPrime(int num) {
        for (int i = 2; i <= sqrt(num); ++i) {
            if (num % i == 0)   return false;
        }

        return true;
    }
};