[RWC] uniqueSubstr

unique-substring.js

/**
 * Sorter function for sorting an array of strings by their length, in descending order
 * @param {string} a
 * @param {string} b
 * @returns positive/negative number representing relative comparison of the two elements
 */
function byLength(a, b) {
	return b.length - a.length;
}

/**
 * Given a string, find its longest substring which contains only 2 unique characters
 * @param {string} str - full string
 */
function uniqueSubstring(str = '') {
	if (str.length <= 2) {
		console.log(str, str.length);
		return;
	}
	
	/** @type {string[]} */
	const substrings = [];

	outer_loop:
	for (let i = 0; i < str.length - 1; i++) {
		// We've already checked previous characters, so now we're seeing if we can find
		// a good substring starting at this current index.
		const stringFromStartingPoint = str.substring(i);

		/** @type {Set<string>} */
		const uniqueCharactersSeen = new Set();

		for (let j = 0; j < stringFromStartingPoint.length; j++) {
			uniqueCharactersSeen.add(stringFromStartingPoint[j]);
			if (uniqueCharactersSeen.size > 2) {
				const candidateSubstring = stringFromStartingPoint.substring(0, j);
				substrings.push(candidateSubstring);
				continue outer_loop;
			}
		}

		substrings.push(stringFromStartingPoint);
	}

	substrings.sort(byLength);
	const [longestSubstring] = substrings;
	console.log(longestSubstring, longestSubstring.length);
}

uniqueSubstring('eceba');
uniqueSubstring('ccaabbb');
uniqueSubstring('abcabcabc');
uniqueSubstring('ababababab');