Roman to Int conversion

[Question]: Convert Roman number into Int, Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

Symbol       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

Example
III = 3
IV = 4

// TC: O(N)
// SC: O(1)
func romanToInt(_ s: String) -> Int {
    var ans = 0, num = 0;
    
    for i in stride(from: s.count - 1, to: -1, by: -1) {
        switch String(s[s.index(s.startIndex, offsetBy: i)]) {
            case "I": num = 1;
            case "V": num = 5;
            case "X": num = 10;
            case "L": num = 50;
            case "C": num = 100;
            case "D": num = 500;
            case "M": num = 1000;
            default: num = 1
        }
        if ans > 4 * num {
            ans -= num
        } else {
            ans += num
        }
    }
    return ans
}

// TC: O(N)
// SC: O(N)
func romanToInt2(_ s: String) -> Int {
      var prev = 0, out = 0
       let dict: [Character:Int] = ["I":1,"V":5,"X":10,"L":50,"C":100,"D":500,"M":1000]

     for i in s {
         let val = dict[i] ?? 0
         let calculation = (val <= prev) ? prev : -prev
         out += calculation
         prev = val
     }
     out += prev
     return out
}

let strRoman = "IV"
let opRoamn = romanToInt(strRoman)
print("roman-- ", opRoamn)// 4