Find largest odd Number/Sequence in String

[Question]: In integer string Return the largest-valued odd integer (as a string) that is a non-empty substring of num, or an empty string "" if no odd integer exists.

A substring is a contiguous sequence of characters within a string.

Example 1:

Input: num = "52"
Output: "5"
Explanation: The only non-empty substrings are "5", "2", and "52". "5" is the only odd number

input 25 – Op- 25.
input 4324 – Op- 43.

// TC: O(N)
// SC: O(N)
func largestOddNumber(_ num: String) -> String {
    if num.isEmpty { return "" }
    // #Approach #1: by iterating from last index Time limit exceed for large input
    var last = num.count - 1
    while last >= 0 {
        let currentChar = String(num[num.index(num.startIndex, offsetBy: last)])
        if Int(currentChar)! % 2 == 0 {
            last -= 1
        } else {// Re
            let range = num.startIndex...num.index(num.startIndex, offsetBy: last)
            return String(num[range])// return the all numbers from the odd index
        }
    }
    return ""
    
    // Approach #2 using clousers
    //    guard let i = num.lastIndex(where: { Int(String($0))! % 2 == 1 }) else { return "" }
    //      return String(num[...i])
}

let ipStr = "4324"
let opStr = largestOddNumber(ipStr)
print("op is ", opStr)//43