#2351 - First Letter to Appear Twice

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String Hash Table Bit Manipulation Counting

Problem Description

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.

Solution

/**
 * @param {string} s
 * @return {character}
 */
var repeatedCharacter = function(s) {
  const set = new Set();

  for (const char of s) {
    if (set.has(char)) return char;
    set.add(char);
  }
};