[Swift]LeetCode159.具有最多两个不同字符的最长子串 $ Longest Substring with At Most Two Distinct Characters

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Given a string S, find the length of the longest substring T that contains at most two distinct characters.
For example:
Given S = “eceba”,
T is “ece” which its length is 3.

---

给定字符串S，找到最长子字符串T的长度，最多包含两个不同的字符。

例如:

给定S = “eceba”,

T是“ece” ，长度为3。

---

class Solution {
    func lengthOfLongestSubstringTwoDistinct(_ s:String) -> Int {
        var left:Int = 0
        var right:Int = -1
        var res:Int = 0
        for i in 1..<s.count
        {
            if s[i] == s[i - 1] {continue}
            if right >= 0 && s[right] != s[i]
            {
                res = max(res, i - left)
                left = right + 1
            }
            right = i - 1
        }
        return max(s.count - left, res) 
    }
}

extension String {        
    //subscript函数可以检索数组中的值
    //直接按照索引方式截取指定索引的字符
    subscript (_ i: Int) -> Character {
        //读取字符
        get {return self[index(startIndex, offsetBy: i)]}
    }
}