Leetcode #287: Find the Duplicate Number

Solution 1: Sorting

func findDuplicate(_ nums: [Int]) -> Int {
    var nums = nums.sorted()
    for i in stride(from: 1, to: nums.count, by: 1) {
        if nums[i] == nums[i-1] {
            return nums[i]
        }
    }
    return -1
}

Solution 2: Set

func findDuplicate(_ nums: [Int]) -> Int {
    var seen = Set<Int>()
    for i in stride(from: 0, to: nums.count, by: 1) {
        if seen.contains(nums[i]) {
            return nums[i]
        }
        seen.insert(nums[i])
    }
    return -1
}