2351. First Letter to Appear Twice.js

// Given a string s consisting of lowercase English letters, return the first letter to appear twice.

// Note:

// A letter a appears twice before another letter b if the second occurrence of a is before the second occurrence of b.
// s will contain at least one letter that appears twice.

// Example 1:

// Input: s = "abccbaacz"
// Output: "c"
// Explanation:
// The letter 'a' appears on the indexes 0, 5 and 6.
// The letter 'b' appears on the indexes 1 and 4.
// The letter 'c' appears on the indexes 2, 3 and 7.
// The letter 'z' appears on the index 8.
// The letter 'c' is the first letter to appear twice, because out of all the letters the index of its second occurrence is the smallest.
// Example 2:

// Input: s = "abcdd"
// Output: "d"
// Explanation:
// The only letter that appears twice is 'd' so we return 'd'.

const repeatedCharacter = s => {
	let letterSet = new Set()

	for (let i = 0; i < s.length; i++) {
		if (letterSet.has(s[i])) return s[i]
		letterSet.add(s[i])
	}
}