LeetCode - Alien Dictionary

AlienDictionary.java

// There is a new alien language which uses the latin alphabet. However, the order among letters are unknown to you. You receive a list of words from the dictionary, where words are sorted lexicographically by the rules of this new language. Derive the order of letters in this language.

// For example,
// Given the following words in dictionary,

// [
//   "wrt",
//   "wrf",
//   "er",
//   "ett",
//   "rftt"
// ]
// The correct order is: "wertf".

// Note:
// You may assume all letters are in lowercase.
// If the order is invalid, return an empty string.
// There may be multiple valid order of letters, return any one of them is fine.

public String alienOrder(String[] words) {
    if(words == null || words.length == 0) return "";
    StringBuilder sb = new StringBuilder();
    Map<Character, Integer> degree = new HashMap<Character, Integer>();
    Map<Character, Set<Character>> toMap = new HashMap<Character, Set<Character>>();
    
    for(String s: words){
        for(char c: s.toCharArray()){
            degree.put(c, 0);
        }
    }
    
    for(int i = 0; i < words.length - 1; i++){
        int length = Math.min(words[i].length(), words[i+1].length());
        for(int j = 0; j < length; j++){
            char cur = words[i].charAt(j), next = words[i+1].charAt(j);
            if(cur == next) continue;
            
            if(toMap.containsKey(cur)){
                if(!toMap.get(cur).contains(next)){
                    toMap.get(cur).add(next);
                    degree.put(next, degree.get(next) + 1);
                } 
            } else {
                Set<Character> set = new HashSet<Character>();
                set.add(next);
                toMap.put(cur, set);
                degree.put(next, degree.get(next) + 1);
            }
            break;
        }
    }
    
    Queue<Character> queue = new LinkedList<Character>();
    for(char c : degree.keySet())
        if(degree.get(c) == 0)
            queue.offer(c);
            
    while(!queue.isEmpty()){
        char cur = queue.poll();
        sb.append(cur);
        if(!toMap.containsKey(cur)) continue;
        for(Character next : toMap.get(cur)){
            degree.put(next, degree.get(next) - 1);
            if(degree.get(next) == 0){
                queue.add(next);
            }
        }
    }
    if(sb.length() != degree.size()) return "";
    return sb.toString();
}

It would have been much easier to understand this code if there was some comments/documentation. Could you please add some text about this logic? Thanks in advance!