leetcode-451-sort-characters-by-frequence.swift

/**

Copyright (c) 2020 David Seek, LLC

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OR OTHER DEALINGS IN THE SOFTWARE.





https://leetcode.com/problems/sort-characters-by-frequency/


Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

Input:
"tree"

Output:
"eert"

Explanation:
'e' appears twice while 'r' and 't' both appear once.
So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.
Example 2:

Input:
"cccaaa"

Output:
"cccaaa"

Explanation:
Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer.
Note that "cacaca" is incorrect, as the same characters must be together.
Example 3:

Input:
"Aabb"

Output:
"bbAa"

Explanation:
"bbaA" is also a valid answer, but "Aabb" is incorrect.
Note that 'A' and 'a' are treated as two different characters.

*/

/**
Big O Annotation
Time complexity O(n log n) because of the sorting overhead,
where n is the amount of elements in nums.
Space complexity O(n) where n is the amount of elements in nums.
*/
func frequencySort(_ s: String) -> String {
        
    // Init a map of characters and their counts
    var map: [Character: Int] = [:]
    
    // Fill the map
    for character in s {
        map[character, default: 0] += 1
    }
    
    // Sort the character : count pairs
    let sorted = map.sorted { element1, element2 in
        return element1.value > element2.value                        
    }
    
    // Init the outut String
    var output = ""
    
    // For each key/value pair...
    for (key, value) in sorted {
        
        // Add value amount of...
        for _ in 0..<value {
            
            // Characters...
            output += String(key)
        }
    }
    
    return output
}