Design Search Autocomplete System

Design a search autocomplete system for a search engine. Users may input a sentence (at least one word and end with a special character '#'). For each character they type except '#', you need to return the top 3 historical hot sentences that have prefix the same as the part of sentence already typed. Here are the specific rules:

1. The hot degree for a sentence is defined as the number of times a user typed the exactly same sentence before.
2. The returned top 3 hot sentences should be sorted by hot degree (The first is the hottest one). If several sentences have the same degree of hot, you need to use ASCII-code order (smaller one appears first).
3. If less than 3 hot sentences exist, then just return as many as you can.
4. When the input is a special character, it means the sentence ends, and in this case, you need to return an empty list.

Your job is to implement the following functions:

The constructor function:

AutocompleteSystem(String[] sentences, int[] times): This is the constructor. The input is historical data. Sentences is a string array consists of previously typed sentences. Times is the corresponding times a sentence has been typed. Your system should record these historical data.

Now, the user wants to input a new sentence. The following function will provide the next character the user types:

List<String> input(char c): The input c is the next character typed by the user. The character will only be lower-case letters ('a' to 'z'), blank space (' ') or a special character ('#'). Also, the previously typed sentence should be recorded in your system. The output will be the top 3 historical hot sentences that have prefix the same as the part of sentence already typed.

Example:
Operation: AutocompleteSystem(["i love you", "island","ironman", "i love leetcode"], [5,3,2,2])
The system have already tracked down the following sentences and their corresponding times:
"i love you" : 5 times
"island" : 3 times
"ironman" : 2 times
"i love leetcode" : 2 times
Now, the user begins another search:

Operation: input('i')
Output: ["i love you", "island","i love leetcode"]
Explanation:
There are four sentences that have prefix "i". Among them, "ironman" and "i love leetcode" have same hot degree. Since ' ' has ASCII code 32 and 'r' has ASCII code 114, "i love leetcode" should be in front of "ironman". Also we only need to output top 3 hot sentences, so "ironman" will be ignored.

Idea: This question is exactly the same as the Search Suggestion System . The subtle difference is that after the input type is finished, the word input must also enter the word frequency statistics. Then the frequency of the sentence needs to be stored in the trienode. Is null, then the future cur is also null,

class AutocompleteSystem {
    private class TrieNode {
        public HashMap<Character, TrieNode> children;
        public HashMap<String, Integer> counts;
        public boolean isword;
        public TrieNode() {
            this.children = new HashMap<Character, TrieNode>();
            this.counts = new HashMap<String, Integer>();
            this.isword = false;
        }
    }
    
    private class Trie {
        private TrieNode root;
        
        public Trie() {
            this.root = new TrieNode();
        }
        
        public void insert(String sentence, int time) {
            TrieNode cur = root;
            for(char c : sentence.toCharArray()) {
                TrieNode next = cur.children.get(c);
                if(next == null) {
                    next = new TrieNode();
                    cur.children.put(c, next);
                }
                cur = next;
                cur.counts.put(sentence, cur.counts.getOrDefault(sentence, 0) + time);
            }
            cur.isword = true;
        }
    }
    
    private class Node {
        private String sentence;
        private int fre;
        public Node(String sentence, int fre) {
            this.sentence = sentence;
            this.fre = fre;
        }
    }
    
    private class NodeComparator implements Comparator<Node> {
        @Override
        public int compare(Node a, Node b) {
            if(a.fre != b.fre) {
                return b.fre - a.fre;
            } else {
                return a.sentence.compareTo(b.sentence);
            }
        }
    }

    private Trie trie;
    private StringBuilder sb;
    private TrieNode cur;
    private String prefix;
    public AutocompleteSystem(String[] sentences, int[] times) {
        trie  = new Trie();
        for(int i = 0; i < sentences.length; i++) {
            trie.insert(sentences[i], times[i]);
        }
        prefix = "";
        sb = new StringBuilder();
        cur = trie.root;
    }
    
    public List<String> input(char c) {
        if(c == '#') {
            trie.insert(sb.toString(), 1);
            sb = new StringBuilder();
            cur = trie.root;
            return new ArrayList<String>();
        }
        sb.append(c);
        // 注意这里，如果下面没有了，那么以后也没有了，cur 赋值为null；
        if(cur == null || !cur.children.containsKey(c)) {
            cur = null;
            return new ArrayList<String>();
        } else {
            cur = cur.children.get(c);
            PriorityQueue<Node> pq = new PriorityQueue<Node>(new NodeComparator());
            for(String sentence: cur.counts.keySet()) {
                pq.add(new Node(sentence, cur.counts.get(sentence)));
            }
            List<String> list = new ArrayList<String>();
            int count = 0;
            while(!pq.isEmpty() && count < 3) {
                Node node = pq.poll();
                list.add(node.sentence);
                count++;
            }
            return list;
        }
    }
}

/**
 * Your AutocompleteSystem object will be instantiated and called as such:
 * AutocompleteSystem obj = new AutocompleteSystem(sentences, times);
 * List<String> param_1 = obj.input(c);
 */