791. Custom Sort String

leetcode791.swift

class Solution {
    func customSortString(_ S: String, _ T: String) -> String {
        
        let sArray = Array(S)
        
        var counter = [Character:Int]()
        
        for char in T {
            counter[char, default:0] += 1
        }
        
        var result = [Character]()
        
        for char in S {
            guard let count = counter[char] else {
                continue
            }
            
            result.append(contentsOf: [Character](repeating:char, count:count))
            counter[char] = nil
        }
        
        for char in T {
            guard let count = counter[char] else {
                continue
            }
            result.append(contentsOf: [Character](repeating:char, count:count))
            counter[char] = nil
        }
        
        return String(result)
    }
}