Refactor code structure to make solutions more readable

Author: Yi Gu

BFS/WordLadder.swift

@@ -10,41 +10,45 @@
 
 class WordLadder {
     func ladderLength(_ beginWord: String, _ endWord: String, _ wordList: [String]) -> Int {
-        guard beginWord.count == endWord.count else {
-            return 0
-        }
-        
-        var queue = [(beginWord, 1)], wordSet = Set<String>(wordList)
+        var wordSet = Set(wordList), wordStepQueue = [(beginWord, 1)]
 
-        while !queue.isEmpty {
-            let (word, step) = queue.removeFirst()
+        while !wordStepQueue.isEmpty {
+            let (currentWord, currentStep) = wordStepQueue.removeFirst()
 
-            if word == endWord {
-                return step
+            if currentWord == endWord {
+                return currentStep
             }
 
-            // transform word
-            for i in 0..<word.count {
-                var wordArray = Array(word)
+            for word in neighbors(for: currentWord, in: wordSet) {
 
-                for char in "abcdefghijklmnopqrstuvwxyz" {
-                    guard char != wordArray[i] else {
-                        continue
-                    }
-                    
-                    wordArray[i] = char
-                    let transformedWord = String(wordArray)
-                    
-                    guard wordSet.contains(transformedWord) else {
-                        continue
-                    }
-                    
-                    wordSet.remove(transformedWord)
-                    queue.append((transformedWord, step + 1))
-                }
+                wordStepQueue.append((word, currentStep + 1))
+                wordSet.remove(word)
             }
         }
 
         return 0
     }
+    
+    private func neighbors(for word: String, in wordSet: Set<String>) -> [String] {
+        var res = [String]()
+        
+        // change character at every offset of the word
+        for i in 0..<word.count {
+            var tempWord = Array(word)
+            
+            for charToReplace in "abcdefghijklmnopqrstuvwxyz" {
+                guard charToReplace != tempWord[i] else {
+                    continue
+                }
+                
+                tempWord[i] = charToReplace
+                let tempWordStr = String(tempWord)
+                if wordSet.contains(tempWordStr) {
+                    res.append(tempWordStr)
+                }
+            }
+        }
+        
+        return res
+    }
 }

DP/EditDistance.swift

@@ -7,27 +7,24 @@
 
 class EditDistance {
     func minDistance(word1: String, _ word2: String) -> Int {
-        let aChars = [Character](word1.characters)
-        let bChars = [Character](word2.characters)
-        let aLen = aChars.count
-        let bLen = bChars.count
+        let word1Chars = Array(word1), word2Chars = Array(word2), m = word1.count, n = word2.count
+        var distances = Array(repeating: Array(repeating: 0, count: n + 1), count: m + 1)
 
-        var dp = Array(count: aLen + 1, repeatedValue:(Array(count: bLen + 1, repeatedValue: 0)))
-        
-        for i in 0...aLen {
-            for j in 0...bLen {
+        for i in 0...m {
+            for j in 0...n {
                 if i == 0 {
-                    dp[i][j] = j
+                    distances[i][j] = j
                 } else if j == 0 {
-                    dp[i][j] = i
-                } else if aChars[i - 1] == bChars[j - 1] {
-                    dp[i][j] = dp[i - 1][j - 1]
+                    distances[i][j] = i
+                } else if word1Chars[i - 1] == word2Chars[j - 1] {
+                    distances[i][j] = distances[i - 1][j - 1]
                 } else {
-                    dp[i][j] = min(dp[i - 1][j - 1], dp[i - 1][j], dp[i][j - 1]) + 1
+                    distances[i][j] = min(distances[i - 1][j - 1], distances[i - 1][j], distances[i][j - 1]) + 1
                 }
+                
             }
         }
 
-        return dp[aLen][bLen]
+        return distances[m][n]
     }
 }