[Swift]LeetCode1071.字符串的最大公因子 | Greatest Common Divisor of Strings

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For strings S and T, we say "T divides S" if and only if S = T + ... + T  (T concatenated with itself 1 or more times)

Return the largest string X such that X divides str1 and X divides str2.

Example 1:

Input: str1 = "ABCABC", str2 = "ABC"
Output: "ABC"

Example 2:

Input: str1 = "ABABAB", str2 = "ABAB"
Output: "AB"

Example 3:

Input: str1 = "LEET", str2 = "CODE"
Output: "" 

Note:

1. 1 <= str1.length <= 1000
2. 1 <= str2.length <= 1000
3. str1[i] and str2[i] are English uppercase letters.

---

对于字符串 S 和 T，只有在 S = T + ... + T（T 与自身连接 1 次或多次）时，我们才认定 “T 能除尽 S”。

返回字符串 X，要求满足 X 能除尽 str1 且 X 能除尽 str2。

示例 1：

输入：str1 = "ABCABC", str2 = "ABC"
输出："ABC"

示例 2：

输入：str1 = "ABABAB", str2 = "ABAB"
输出："AB"

示例 3：

输入：str1 = "LEET", str2 = "CODE"
输出："" 

提示：

1. 1 <= str1.length <= 1000
2. 1 <= str2.length <= 1000
3. str1[i] 和 str2[i] 为大写英文字母 

---

Runtime: 12 ms

Memory Usage: 21.1 MB

class Solution {
    func gcdOfStrings(_ str1: String, _ str2: String) -> String {
        var str1 = str1
        var str2 = str2
        let arrStr1:[Character] = Array(str1)
        let arrStr2:[Character] = Array(str2)
        var n:Int = str1.count
        var m:Int = str2.count

        for i in stride(from:n,to:0,by:-1)
        {
            if (n % i != 0) || (m % i != 0) {continue}
            var flag:Bool = false
            for j in 0..<m
            {
                if arrStr2[j] != arrStr1[j % i]
                {
                    flag = true
                    break
                }
            }
            if flag {continue}
            for j in 0..<n
            {
                if arrStr1[j] != arrStr1[j % i]
                {
                    flag = true
                    break
                }
            }
            if flag {continue}
            return str1.subString(0, i)
        }
        return String()
    }
}

extension String {
    // 截取字符串：指定索引和字符数
    // - begin: 开始截取处索引
    // - count: 截取的字符数量
    func subString(_ begin:Int,_ count:Int) -> String {
        let start = self.index(self.startIndex, offsetBy: max(0, begin))
        let end = self.index(self.startIndex, offsetBy:  min(self.count, begin + count))
        return String(self[start..<end])
    }
}

---

56ms

class Solution {
    func gcdOfStrings(_ str1: String, _ str2: String) -> String {
        let shorterStr: String
        let longerStr: String
        if str1.count > str2.count {
            longerStr = str1
            shorterStr = str2
        } else {
            longerStr = str2
            shorterStr = str1
        }
        
        for i in stride(from: shorterStr.count, to: 1, by: -1) where shorterStr.count % i == 0 && longerStr.count % i == 0 {
            let range = shorterStr.startIndex..<shorterStr.index(shorterStr.startIndex, offsetBy: i)
            let t = String(shorterStr[range])
            guard shorterStr == String(repeating: t, count: shorterStr.count / i) else { continue }
            guard longerStr == String(repeating: t, count: longerStr.count / i) else { continue }
            return t
        }
        return ""
    }
}

---

120ms

class Solution {
  func gcdOfStrings(_ str1: String, _ str2: String) -> String {
    let (long, short) = str1.count > str2.count ? (str1, str2) : (str2, str1)
    let common = long.commonPrefix(with: short)
    var mu = [Substring]()
    for i in 1...short.count {
      if short.count % i == 0 {
        mu.append(short.prefix(short.count / i))
      }
    }
    for m in mu {
      let s = String(m)
      if long == String(repeating: s, count: long.count/s.count) &&
        short == String(repeating: s, count: short.count/s.count) {
        return s
      }
    }
    return ""
  }
}

---

184ms

class Solution {
  func gcdOfStrings(_ str1: String, _ str2: String) -> String {
    let shortestString = str1 > str2 ? str2 : str1
    let longestString = str1 <= str2 ? str2 : str1
    print(shortestString)
    guard longestString.range(of: shortestString) != nil else { return "" }
    if longestString.count % shortestString.count == 0 {
      let timesGreater = longestString.count / shortestString.count
      return isRepeatative(substring: shortestString, in: longestString, times: timesGreater) ? shortestString : ""
    } else {
      let substrs = getSubstring(shortestString)
      var longestSubstring = ""
      for substr in substrs {
        if longestString.count % substr.count == 0 {
          let times = longestString.count / substr.count
          if isRepeatative(substring: substr, in: longestString, times: times) && substr.count > longestSubstring.count {
            longestSubstring = substr
          }
        }
      }
      return longestSubstring
    }
  }

  func isRepeatative(substring: String, in string: String, times: Int) -> Bool {
    var str = ""
    for _ in 0..<times {
      str.append(substring)
    }
    return string == str
  }

  func getSubstring(_ string: String) -> [String] {
    let startIndex = string.startIndex
    var substrings = [string]
    for i in 2..<string.count {
      let stopIndex = String.Index.init(encodedOffset: string.count / i)
      let substring = String(string[startIndex..<stopIndex])
      if isRepeatative(substring: substring, in: string, times: i) {
        substrings.append(substring)
      }
    }
    return substrings
  }
}