Missing Number" problem — a common coding question on LeetCode (#268) and interviews at companies like Amazon.

🧩 Problem Statement (LeetCode 268)

You are given an array containing n distinct numbers taken from the range 0 to n.
Return the one number that is missing from the array.

🧠 Example:

Input: [3, 0, 1]
Output: 2

✅ Swift Solutions

🔹 1. Using Sum Formula (O(n) time, O(1) space)

func missingNumber(_ nums: [Int]) -> Int {
    let n = nums.count
    let expectedSum = n * (n + 1) / 2
    let actualSum = nums.reduce(0, +)
    return expectedSum - actualSum
}

🧠 Explanation:

The sum of numbers from 0...n is n(n+1)/2.

Subtract actual array sum from expected sum to get the missing number.

🔹 2. Using XOR (No Extra Space, O(n) time)

func missingNumber(_ nums: [Int]) -> Int {
    var result = nums.count
    for (i, num) in nums.enumerated() {
        result ^= i ^ num
    }
    return result
}

🧠 Explanation:

XOR all indices and values; missing one will cancel out and remain.

🔹 3. Using Set (Extra Space O(n))

func missingNumber(_ nums: [Int]) -> Int {
    let set = Set(nums)
    for i in 0...nums.count {
        if !set.contains(i) {
            return I
        }
    }
    return -1
}

✅ Example Call:

let arr = [0, 1, 3]
print(missingNumber(arr))  // Output: 2