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.)

Thought

重複字符串的長度和肯定是一個子字符串的整數倍¹²。

Solution

public class Solution {
    public boolean repeatedSubstringPattern(String s) {
        int n = s.length();
        int[] next = new int[n];
        int i = 1, j = 0;
        while (i < n) {
            if (s.charAt(i) == s.charAt(j)) next[i++] = ++j;
            else if (j == 0) next[i++] = 0;
            else j = next[j - 1];
        }

        return next[n - 1] != 0 && n % (n - next[n - 1]) == 0;
    }
}

¹. [LeetCode] Repeated Substring Pattern 重複子字符串模式 ↩

². C++ O(n) using KMP, 32ms, 8 lines of code with brief explanation ↩