03_PrefixSum

func prefixSum(_ array: [Int]) -> [Int] {
    var result: [Int] = []
    var sum: Int = 0
    for num in array {
        sum += num
        result.append(sum)
    }
    return result
}

let nums = (0...5).map { $0 }
print("Original Array: \(nums)")
print("Prefix Sum Array: \(prefixSum(nums))")

// ------------------------------------------------------------------------------------------------------------------------
//Indexed based

func prefixSumIndexed(_ array: [Int]) -> [Int] {
    var result = [Int]()
    result.reserveCapacity(array.count)
    
    for (index, _ ) in array.enumerated() {
       if index == 0 {
           result.append(array[0])
           continue
        }
        result.append(array[index] + result[index - 1])
    }
    return result
}


print("Original Array: \(nums)")
print("Prefix Sum Array: \(prefixSumIndexed(nums))")


// ------------------------------------------------------------------------------------------------------------------------
// Pointer based prefix sum
func buildPrefix(input: [Int]) -> [Int] {
    var expectation = [Int]()
    
    var leftIndex = input.startIndex
    let endIndex = input.endIndex
    
    var summation = 0
    while leftIndex < endIndex {
        summation += input[leftIndex]
        expectation.append(summation)
        leftIndex += 1
    }
    
    return expectation
}

print(prefixSum(nums))