Swift_集合 let inde = arr.firstInd

let inde = arr.firstIndex(of element: Element)

-> Int?

let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
let inde = streets.firstIndex(of: "Evarts")    // 4
let streetsSlice = streets[2...]
// ["Channing", "Douglas", "Evarts"]
let inde0 = streetsSlice.firstIndex(of: "Evarts")    // 4
let inde1 = streetsSlice.firstIndex(of: "aa")  // nil
print(inde0 as Any)
print(inde1 as Any)
// Log inde0 == 4
// Log inde1 == nil