# Find missing number from first N natural numbers.

Question: Find the missing number in first n Natural numbers…

``````// Approach 1: Using Summation
// TC:O(n)
// SC:O(1)
func findMissingNumber1(arr: [Int]) ->Int {
let len = arr.count + 1
var summationOfFirstN = len*(len+1)/2
for value in arr {
summationOfFirstN -= value
}
return summationOfFirstN
}

// Approach 2: Using Summation
// TC:O(n)
// SC:O(1)
//To avoid integer overflow, pick one number from the range [1, N] and subtract a number from the given array (don’t subtract the same number twice)

func findMissingNumber2(arr: [Int]) ->Int {
var total: Int = 1
var arrCount = arr.count + 1

for i in 2..<arrCount {
total += i;
total = total - arr[i - 2];
}
}

// Approach 3: Using XOR
// TC:O(2n)
// SC:O(1)
func findMissingNumber3(arr: [Int]) ->Int {

var arrCount = arr.count
// For xor of all the elements in array
var x1 = arr[0];

// For xor of all the elements from 1 to n+1
var x2 = 1;

for i in 1..<arrCount-1 {
x1 = x1 ^ arr[i]
}
for i in 2..<arrCount+1 {
x2 = x2 ^ i
}
return x1 ^ x2
}
var arrayMiss: [Int] = [1,2,3,5]
print("Missing numbers is-->", findMissingNumber1(arr: arrayMiss))// 4
print("Missing numbers is-->", findMissingNumber2(arr: arrayMiss))// 4
print("Missing numbers is-->", findMissingNumber3(arr: arrayMiss))//4``````