This problem was asked by Stripe.

Given an array of integers, find the first missing positive integer in linear time and constant space. In other words, find the lowest positive integer that does not exist in the array. The array can contain duplicates and negative numbers as well.

For example, the input [3, 4, -1, 1] should give 2. The input [1, 2, 0] should give 3.

You can modify the input array in-place.

### Solution

I wrote the solutions in Swift. Feel free to comment with your solution in the language of your choice.

```
var myArray = [3, 4, -1, 1]
func firstMissingPositiveInt(_ :[Int]) -> Int {
var smallest = myArray[0], largest = myArray[0]
var missing = Int()
var included = false
// Find smallest and largest Int
for integer in myArray {
if integer < smallest {
smallest = integer
}
if integer > largest {
largest = integer
}
}
// If missing Int smaller than smallest
if smallest > 1 {
missing = smallest - 1
// If both the smallest and largest Int are less than 0
} else if smallest < 1 && largest < 1 {
missing = 1
// If missing Int greater than smallest
} else {
smallest = smallest < 1 ? 1 : smallest
for value in smallest...largest+1 {
for integer in myArray {
if value == integer { included = true }
}
if !included { missing = value; break }
if included { included = false }
}
}
return missing
}
firstMissingPositiveInt(myArray)
print("The first missing Int is \(firstMissingPositiveInt(myArray)).")
```

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