<pre class="shiki github-dark shaku"><code>/** * @param {string} str * @return {string | null} */function firstDuplicate(str) { // your code here let map = new Map(); let i =0; while(i &lt; str.length) { let char = str[i]; if(!map.has(char)) { map.set(char, 1); } else { return char; } i++; } return null;}</code></pre>

Time: O(n) Space: O(n)

Apple

Given a string which might have duplicate letters, write a function to find the first duplicate. firstDuplicate('abca')// 'a'firstDuplicate('abcdefe')…