You are given an integer array nums with the following properties: nums.length == 2 * n. nums contains n + 1 unique elements. Exactly one element of nums is repeated n times. Return the element that is repeated n times.

repeatedNTimes.js

/**
 * @param {number[]} nums
 * @return {number}
 */
var repeatedNTimes = function(nums) {
    // We'll use a Set to track which numbers we've seen.
    const seen = new Set();
    
    // Walk through each number in the array
    for (let num of nums) {
        // If we've seen this number before, it must be the one
        // that is repeated n times (because only one element repeats).
        if (seen.has(num)) {
            return num;
        }
        
        // Otherwise, record it as seen and continue.
        seen.add(num);
    }
    
    // Given the problem guarantees, we should always have returned
    // inside the loop. This is just a fallback.
    return -1;
};