Find K Largest element

// Find kth largest element in an array
export function findKthLargest(a: number[], k: number): number {
    const i = a.length - k
    if (i < 0 || i >= a.length) {
        // out of range
		return Infinity
	}
	find(a, 0, a.length - 1, i)
    return a[i]
}

function swap(a: number[], i: number, j: number) {
	const tmp = a[i]
	a[i] = a[j]
	a[j] = tmp
}

function find(a: number[], low: number, high: number, i: number): number {
    if (low === high) return low

    const pi = partition(a, low, high)
    if (pi === i) return pi
    if (pi > i) return find(a, low, pi-1, i)
    return find(a, pi+1, high, i)
}

function partition(a: number[], low: number, high: number): number {
	const pivot = a[high]

    // find first position i where a[i] >= pivot (could be pivot position itself)
	let i = low
	while (a[i] < pivot) {
		i++
	}

    // up to the position prior to the pivot, if found element smaller than pivot then move it to the left
	for (let j = i + 1; j < high; j++) {
		if (a[j] < pivot) {
			swap(a, i, j)
            i++
		}
	}

    // swap pivot with i
    if (i < high) swap(a, i, high)

	return i
}