[Swift]LeetCode43. 字符串相乘 | Multiply Strings

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Given two non-negative integers num1 and num2 represented as strings, return the product of num1and num2, also represented as a string.

Example 1:

Input: num1 = "2", num2 = "3"
Output: "6"

Example 2:

Input: num1 = "123", num2 = "456"
Output: "56088"

Note:

1. The length of both num1 and num2 is < 110.
2. Both num1 and num2 contain only digits 0-9.
3. Both num1 and num2 do not contain any leading zero, except the number 0 itself.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

---

给定两个以字符串形式表示的非负整数 num1 和 num2，返回 num1 和 num2 的乘积，它们的乘积也表示为字符串形式。

示例 1:

输入: num1 = "2", num2 = "3"
输出: "6"

示例 2:

输入: num1 = "123", num2 = "456"
输出: "56088"

说明：

1. num1 和 num2 的长度小于110。
2. num1 和 num2 只包含数字 0-9。
3. num1 和 num2 均不以零开头，除非是数字 0 本身。
4. 不能使用任何标准库的大数类型（比如 BigInteger）或直接将输入转换为整数来处理。

---

56ms

class Solution {
  func integerArray(text: String) -> [Int] {
    let characterArray = Array(text)
    var integerArray = [Int]()
    for character in characterArray {
      let unitScalar = character.unicodeScalars.first!.value - Unicode.Scalar("0")!.value
      integerArray.append(Int(unitScalar))
    }

    return integerArray
  }


  func multiply(_ num1: String, _ num2: String) -> String {
    if num1 == "0" || num2 == "0" {
        return "0"
    }
    let num1Array = integerArray(text: num1)
    let num2Array = integerArray(text: num2)
    let maxCount = num1Array.count + num2Array.count - 1
    var productArray = [Int](repeating: 0, count: maxCount)
    var i = 0
    for number1 in num1Array {
      var j = i
      for number2 in num2Array {
        productArray[j] += number1 * number2
        j += 1
      }
      i += 1
    }

    var characterArray = [Character]()
    let lastIndex = productArray.count - 1
    var carry = 0
    for k in 0...lastIndex {
      let l = lastIndex - k
      var value = productArray[l] + carry
      if value >= 10 {
        carry = value / 10
        value = value % 10
      } else {
        carry = 0
      }

      let character: Character = Character(String(value))
      characterArray.insert(character, at: 0)
    }
    if carry > 0 {
      let character: Character = Character(String(carry))
      characterArray.insert(character, at: 0)
    }

    return String(characterArray)
  }
}

---

72ms

class Solution {
    func multiply(_ num1: String, _ num2: String) -> String {
        if num1 == "0" || num2 == "0" {
            return "0"
        }
        
        var result = String()
        
        var num1Arr = Array(num1), num2Arr = Array(num2)
        let n1 = num1.count, n2 = num2.count
        var k = n1 + n2 - 2, carry = 0
        var v = [Int](repeating: 0, count: n1 + n2)
        let zeroValue = Int("0".unicodeScalars.first!.value)
        for i in 0..<n1 {
            for j in 0..<n2 {
                let num1Value = Int(num1Arr[i].unicodeScalars.first!.value)
                let num2Value = Int(num2Arr[j].unicodeScalars.first!.value)
                v[k - i - j] += (num1Value - zeroValue) * (num2Value - zeroValue)
            }
        }
        
        for i in 0..<n1 + n2 {
            v[i] += carry
            carry = v[i] / 10
            v[i] %= 10
        }
        var i = n1 + n2 - 1
        while v[i] == 0 {
            i -= 1
        }
        if i < 0 {
            return "0"
        }
        while i >= 0 {
            let c = Character(UnicodeScalar(zeroValue + v[i])!)
            result.append(String(c))
            i -= 1
        }
        
        return result
    }
}

---

88ms

class Solution {
    func multiply(_ num1: String, _ num2: String) -> String {
        let left = num1.map({ Int(String($0))! })
        let right = num2.map({ Int(String($0))! })

        let m = left.count
        let n = right.count
        var result = [Int](repeating: 0, count: m + n)

        for i in (0..<m).reversed() {
            for j in (0..<n).reversed() {
                let mul = left[i] * right[j]
                let pos1 = i + j
                let pos2 = i + j + 1
                let sum = mul + result[pos2]

                result[pos1] += sum / 10
                result[pos2] = sum % 10
            }
        }

        var answer = ""
        for i in result {
            if answer.count == 0 && i == 0 {
                continue
            }

            answer += "\(i)"
        }
        
        if answer.count == 0 {
            return "0"
        }

        return answer
    }
}

---

100ms

class Solution {
    func multiply(_ num1: String, _ num2: String) -> String {
        let num1Array = Array(num1).map {Int(String($0))}
        let num2Array = Array(num2).map {Int(String($0))}
        var ansArray = Array(repeatElement(0,count:num1Array.count+num2Array.count))
        var step = 0
        for index1 in stride(from:num1Array.count-1, through:0, by:-1) {
            for index2 in stride(from:num2Array.count-1, through:0, by:-1) {
                let value = num1Array[index1]! * num2Array[index2]!
                ansArray[step+num2Array.count-1-index2] = ansArray[step+num2Array.count-1-index2] + value
                var carrier = ansArray[step+num2Array.count-1-index2] / 10
                ansArray[step+num2Array.count-1-index2] = ansArray[step+num2Array.count-1-index2] - (carrier*10)
                ansArray[step+num2Array.count-1-index2+1] = ansArray[step+num2Array.count-1-index2+1] + carrier
            }
            step = step + 1
        }
        
        ansArray.reverse()
        var lastNonZeroIndex = 0
        for value in ansArray {
            if value==0 {
                lastNonZeroIndex = lastNonZeroIndex + 1
            } else {
                break
            }
        }

        for count in stride(from:lastNonZeroIndex, to:0, by:-1) {
            ansArray.removeFirst()
        }

        if ansArray.count == 0 {
            return "0"
        }
        return (ansArray.map {String($0)}).joined(separator: "")
    }
}

---

168ms

class Solution {
    
    private let charToInt : [Character : Int] = 
    [
        "0" : 0, "1" : 1, "2" : 2, "3" : 3, "4" : 4,
        "5" : 5, "6" : 6, "7" : 7, "8" : 8, "9" : 9
    ]
    
    func multiply(_ num1: String, _ num2: String) -> String {
        let chars1 = Array(num1.characters)
        let chars2 = Array(num2.characters)
        
        var num1Count = num1.count - 1
        var num2Count = num2.count - 1
        var multiply = 0
        let size = num2.count + num1.count
        var result = [Int](repeating: 0, count: size)
        
        for i in stride(from: num1Count, to: -1, by: -1) {
            for j in stride(from: num2Count, to: -1, by: -1) {
                var n1 = i >= 0 ? charToInt[chars1[i]] : 0
                var n2 = j >= 0 ? charToInt[chars2[j]] : 0

                let pos1 = i+j+1
                let pos2 = i+j
                multiply = n1! * n2! + result[pos1]
                
                result[pos1] =  multiply%10
                result[pos2] = result[pos2] + (multiply/10)
            }
        }

        var resultString = ""
        var removeZero = false
        for num in result {
            if num != 0 {
                removeZero = true
            }
            if removeZero {
                resultString = resultString + String(num)
            }
        }
        if resultString == "" {
            return "0"
        }
        
        return resultString

    }
}

---

228ms

class Solution {
    func multiply(_ num1: String, _ num2: String) -> String {
        let m = num1.count
        let n = num2.count
        let zero = Character("0").ascii
        var pos = [Int](repeating: 0, count: m + n)
        var res = [String]()

        for i in (0..<m).reversed() {
            for j in (0..<n).reversed() {
                let mul = (num1[i].ascii - zero) * (num2[j].ascii - zero)
                let p1 = i + j
                let p2 = i + j + 1
                let sum = mul + pos[p2]
                pos[p1] += sum / 10
                pos[p2] = sum % 10
            }
        }

        for p in pos {
            if !(res.count == 0 && p == 0) {
                res.append(String(p))
            }
        }

        return res.count == 0 ? "0" : res.joined()
    }
}

extension Character {
    var ascii: Int {
        get {
            let s = String(self).unicodeScalars
            return Int(s[s.startIndex].value)
        }
    }
    
    func isDigit() -> Bool {
        return self >= "0" && self <= "9"
    }
}

extension String {
    subscript (i: Int) -> Character {
        return self[index(startIndex, offsetBy: i)]
    }
}