Given two strings S and T, determine if they are both one edit distance apart.

給定兩個字串S和T，確定它們是否都是是一步變換得到的。

class Solution {
    func isOneEditDistance(_ s:String, _ t:String) -> Bool{
        var num:Int = min(s.count, t.count)
        for i in 0..<num
        {
            
 
if s.count == t.count
            {
                return s.substring(i + 1) == t.substring(i + 1)
            }
            else if s.count < t.count
            {
                return s.substring(i) == t.substring(i + 1)
            }
            else
            {
                
 
return s.substring(i + 1) == t.substring(i)
            }
        }
        return abs(s.count - t.count) == 1
    }
}

extension String {
    // 擷取字串：從index到結束處
    // - Parameter index: 開始索引
    // - Returns: 子字串
    func substring(_ index: Int) -> String {
        let theIndex = self.index(self.endIndex, offsetBy: index - self.count)
        
 
return String(self[theIndex..<endIndex])
    }
}