Repeated Substring Pattern, searching for a repeatable substring within a string.

repeated-substring.js

/**
 * @param {string} str
 * @return {boolean}
 */
var repeatedSubstringPattern = function(str) {
    var result = false;

    var sameCount = 1;
    for (var i=1; i<str.length; i++) {
        if (str[i] === str[0]) {
            sameCount++;
        }
        else {
            break;
        }
    }

    if (sameCount > 1 && sameCount === str.length) {
        result = true;
    }
    else {
        // Start the search at the 3rd letter (since we already covered cases for 1 and 2 characters above).
        for (var i=2; i<=str.length / 2; i++) {
            // The substring must be a divsor of the string length. This dramatically speeds up the search!
            if (str.length % i === 0) {
                // Get the substring.
                var sub = str.substring(0, i);
                skip = false;
    
                // Check if the substring is found at each nth index in the string (divisor of the length).
                for (var j=sub.length; j<=str.length - sub.length; j+=sub.length) {
                    if (str.indexOf(sub, j) !== j) {
                        // Found the substring, but not at the next divsior index.
                        result = false;
                        skip = true;
                        break;
                    }
                }
    
                // If we got this far, and j reached the end of the string evenly, we have a match!
                if (!skip && j === str.length) {
                    result = true;
                    break;
                }
            }
        }
    }

    return result;
};

zexample.txt

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.)

There is a very efficient algorithm for this.

var isStringRepeating = s => (s + s).indexOf(s,1) !== s.length;