Swift 算法实现之逐字翻转字符串

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李峰峰博客

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一、概述

翻转字符串在字符串算法中算是比较常见的，而且被很多公司用作笔试题。”逐字翻转字符串”是翻转字符串的翻版，也是之前Google的面试题，原题是这样的：

Given an input string, reverse the string word by word.
A word is defined as a sequence of non-space characters.
The input string does not contain leading or trailing spaces and the words are always separated by a single space.
For example,
Given s = "the sky is blue",
return "blue is sky the".
Could you do it in-place without allocating extra space?

简而言之就是：”the sky is blue”—>”blue is sky the”

所以，对于本文，要解决的算法是：

逐字翻转字符串，例如："the sky is blue"—>"blue is sky the"

接下来看下实现思路和代码。

 

二、实现思路及代码

既然是字符串翻转的翻版，我们就可以利用之前翻版字符串的思路去解决就可以了，不过这道题要有两次翻转：

第一次翻转，整体翻转：”the sky is blue” -> “eulb si yks eht”

第二次翻转，单词翻转：”eulb si yks eht” -> “blue is sky the”

所以，首先可以实现一个可以翻转局部和全部字符串的算法，传入字符数组、startIndex 和 endIndex ，其中 startIndex 和 endIndex 分别为要翻转的字符串的起始下标和结束下标，也就是要翻转 startIndex 和 endIndex 之间（包含）的字符，代码如下：

func _reverseStr( _ chars:inout [Character], _ startIndex:Int, _ endIndex:Int){
    
    var startIndex = startIndex
    var endIndex = endIndex
    
    if startIndex <= endIndex {
        
        let tempChar = chars[endIndex]
        chars[endIndex] = chars[startIndex]
        chars[startIndex] = tempChar
        
        startIndex += 1
        endIndex -= 1
        
        _reverseStr(&chars,startIndex,endIndex)
        
    }
    
}

之后就可以利用上面的算法去完成前面说的两次翻转：

func reverseWords(_ str:String) -> String{

    var chars = [Character](str.characters)
    
    //首先翻转整个字符串所有字符,"the sky is blue" -> "eulb si yks eht"
    _reverseStr(&chars,0,chars.count-1)
    
    //然后翻转每个单词中的字符，"eulb si yks eht" -> "blue is sky the"
    var startIndex = 0
    for endIndex in 0 ..< chars.count {
        if endIndex == chars.count - 1 || chars[endIndex + 1] == " " {
            _reverseStr(&chars, startIndex, endIndex)
            startIndex = endIndex + 2
        }
    }
    
    return String(chars)
}

 

完整算法代码：

//翻转指定范围的字符
func _reverseStr( _ chars:inout [Character], _ startIndex:Int, _ endIndex:Int){
    
    var startIndex = startIndex
    var endIndex = endIndex
    
    if startIndex <= endIndex {
        
        let tempChar = chars[endIndex]
        chars[endIndex] = chars[startIndex]
        chars[startIndex] = tempChar
        
        startIndex += 1
        endIndex -= 1
        
        _reverseStr(&chars,startIndex,endIndex)
        
    }
    
}

//逐字翻转字符串
func reverseWords(_ str:String) -> String{

    var chars = [Character](str.characters)
    
    //首先翻转整个字符串所有字符,"the sky is blue" -> "eulb si yks eht"
    _reverseStr(&chars,0,chars.count-1)
    
    //然后翻转每个单词中的字符，"eulb si yks eht" -> "blue is sky the"
    var startIndex = 0
    for endIndex in 0 ..< chars.count {
        if endIndex == chars.count - 1 || chars[endIndex + 1] == " " {
            _reverseStr(&chars, startIndex, endIndex)
            startIndex = endIndex + 2
        }
    }
    
    return String(chars)
}

reverseWords("the sky is blue") //return "blue is sky the"

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