Find a duplicate in an array | Pigeonhole principle (typescript)

My solution to a “Daily Coding Problem” that I received in my mail today.

You are given an array of length n + 1 whose elements belong to the set {1, 2, ..., n}. By the pigeonhole principle, there must be a duplicate. Find it in linear time and space.

Daily coding problem solution in Typescript,

oneSixtyFour(array: number[]): number | boolean{
    //we know for sure the array will be length + 1
    //no need to do null checks?
    let uniqueVals = new Set<number>();
    array.forEach(num => {
        if(uniqueVals.has(num)) {
            //we are assuming we return the first 
            //duplicate value we find
            return num;
        } else {
            uniqueVals.add(num);
        }
    });
    return false;
} How about an alternative?

oneSixtyFourSort(array: number[]): number | boolean{
    //we know for sure the array will be length + 1
    let uniqueVals = new Set<number>();
    array.sort();
    for(let i=1; i < array.length; i++ ){
        if(array[i] == array[i-1]) {
            return array[i];
        }
    }
    return false;
}

Ok the second function is just an alternative solution, I reckon it’s slightly cleaner and better looking.

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