Repeated Substring Pattern

LeetCode 459 | Difficulty: Easy

Easy

Problem Description

Given a string s, check if it can be constructed by taking a substring of it and appending multiple copies of the substring together.

Example 1:

Input: s = "abab"
Output: true
Explanation: It is the substring "ab" twice.

Example 2:

Input: s = "aba"
Output: false

Example 3:

Input: s = "abcabcabcabc"
Output: true
Explanation: It is the substring "abc" four times or the substring "abcabc" twice.

Constraints:

- `1 <= s.length <= 10^4`

- `s` consists of lowercase English letters.

Topics: String, String Matching

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Approach

String Processing

Consider character frequency counts, two-pointer approaches, or building strings efficiently. For pattern matching, think about KMP or rolling hash. For palindromes, expand from center or use DP.

When to use

Anagram detection, palindrome checking, string transformation, pattern matching.

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Solutions

Solution 1: C# (Best: 384 ms)

Metric	Value
Runtime	384 ms
Memory	N/A
Date	2018-04-04

Solution

public class Solution {
    public bool RepeatedSubstringPattern(string s) {
       int n = s.Length;
    StringBuilder sb = new StringBuilder();
    for (int i = 1; i <= n/2; i++)
    {
        if(n%i!=0) continue;
        sb.Clear();
        string substr = s.Substring(0,i);
        while(sb.Length < n)
        {
            sb.Append(substr);          
        }
        if(sb.ToString().Equals(s)) return true;
    }
    return false; 
    }
}

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Complexity Analysis

Approach	Time	Space
Solution	$O(n)$	$O(1) to O(n)$

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Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.