swift - 如何修复 fatal error : Can't form a Character from an empty String

Question

我是 Swift 的新手，调试花了我 1 小时，但我仍然不知道为什么会失败。

这是leetcode的一道题:反转字符串:

输入:“你好”

输出:“olleh”

还有我的代码:

import Foundation
class Solution {
    func reverseString(_ s: String) -> String {
        // initialize a new null string
        var answer = ""
        // get the length of the input string
        var lengthOfString = s.characters.count
        // add the character into the new string
        for nums in 0...(lengthOfString-1){
            var index = s.index(s.startIndex, offsetBy: lengthOfString)
            answer.append(s[index])
            lengthOfString-=1
        }
        return answer
    }
}

let a = Solution()
let b = a.reverseString("hello")
print(b)

这是我错误的图片:

Answer 1

问题出在这个 block 中:

for nums in 0...(lengthOfString-1){
    var index = s.index(s.startIndex, offsetBy: lengthOfString)
    answer.append(s[index])
    lengthOfString-=1
}

1. 应该使用0..<lengthOfString范围。
2. 您正在更改 lengthOfString 的值，这是一种非常糟糕的方式。

这是一个反转字符串的示例:

extension String {

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

    subscript (i: Int) -> String {
        return String(self[i] as Character)
    }

    func reverse() -> String {

        let count = characters.count
        if count == 0 { return "" }

        var result = ""
        for i in 0..<count {
            result = self[i] + result
        }

        return result

    }

}

let s = "hello"
s.reverse() // print: olleh