MedianTwoSortedArrays.swift

/**
 * Question Link: https://leetcode.com/problems/median-of-two-sorted-arrays/
 * Primary idea: Search kth value of both arrays and then compare, each time 
 *               remove half of them until index is 0
 *
 * Time Complexity: O(log(m + n)), Space Complexity: O(1)
 */

class MedianTwoSortedArrays {
    func findMedianSortedArrays(_ nums1: [Int], _ nums2: [Int]) -> Double {
        let m = nums1.count
        let n = nums2.count
        
        return (findKth(nums1, nums2, (m + n + 1) / 2) + findKth(nums1, nums2, (m + n + 2) / 2)) / 2
    }
    
    private func findKth(_ nums1: [Int], _ nums2: [Int], _ index: Int) -> Double {
        let m = nums1.count
        let n = nums2.count
        
        guard m <= n else {
            return findKth(nums2, nums1, index)
        }
        guard m != 0 else {
            return Double(nums2[index - 1])
        }
        guard index != 1 else {
            return Double(min(nums1[0], nums2[0]))
        }
        
        let i = min(index / 2, m)
        let j = min(index / 2, n)
        
        if nums1[i - 1] < nums2[j - 1] {
            return findKth(Array(nums1[i..<m]), nums2, index - i)
        } else {
            return findKth(nums1, Array(nums2[j..<n]), index - j)
        }
    }
}