題目連結: https://leetcode.com/problems/excel-sheet-column-title/
func convertToTitle(_ columnNumber: Int) -> String {
var tmp = columnNumber
let baseVal = UnicodeScalar("A").value
var result = ""
while tmp > 0 {
let val = (tmp - 1) % 26
result = String(UnicodeScalar(baseVal + UInt32(val))!) + result
tmp = (tmp - 1) / 26
}
return result
}