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