1. ReturnFirstRecurringCharacter

function firstRecurringCharacter(arr) {
    const seen = new Set();

    for (let num of arr) {
        if (seen.has(num)) {
            return num; // Found the first recurring character
        }
        seen.add(num);
    }

    return undefined; // No recurring character found
}
console.log(firstRecurringCharacter([2,5,1,2,3,5,1,2,4])); // Output: 2
console.log(firstRecurringCharacter([2,1,1,2,3,5,1,2,4])); // Output: 1
console.log(firstRecurringCharacter([2,3,4,5]));           // Output: undefined

Time and Space Complexity:
Time Complexity: O(n) — only one pass through the array.

Space Complexity: O(n) — in the worst case, we store all unique elements in the Set