[LeetCode] 1268. Search Suggestions System

You are given an array of strings products and a string searchWord.

Design a system that suggests at most three product names from products after each character of searchWord is typed. Suggested products should have common prefix with searchWord. If there are more than three products with a common prefix return the three lexicographically minimums products.

Return a list of lists of the suggested products after each character of searchWord is typed. 

Example 1:

Input: products = ["mobile","mouse","moneypot","monitor","mousepad"], searchWord = "mouse"
Output: [
["mobile","moneypot","monitor"],
["mobile","moneypot","monitor"],
["mouse","mousepad"],
["mouse","mousepad"],
["mouse","mousepad"]
]
Explanation: products sorted lexicographically = ["mobile","moneypot","monitor","mouse","mousepad"]
After typing m and mo all products match and we show user ["mobile","moneypot","monitor"]
After typing mou, mous and mouse the system suggests ["mouse","mousepad"]

Example 2:

Input: products = ["havana"], searchWord = "havana"
Output: [["havana"],["havana"],["havana"],["havana"],["havana"],["havana"]]

Example 3:

Input: products = ["bags","baggage","banner","box","cloths"], searchWord = "bags"
Output: [["baggage","bags","banner"],["baggage","bags","banner"],["baggage","bags"],["bags"]]

Constraints:

• 1 <= products.length <= 1000
• 1 <= products[i].length <= 3000
• 1 <= sum(products[i].length) <= 2 * 104
• All the strings of products are unique.
• products[i] consists of lowercase English letters.
• 1 <= searchWord.length <= 1000
• searchWord consists of lowercase English letters.

搜索推荐系统。

给你一个产品数组 products 和一个字符串 searchWord ，products  数组中每个产品都是一个字符串。

请你设计一个推荐系统，在依次输入单词 searchWord 的每一个字母后，推荐 products 数组中前缀与 searchWord 相同的最多三个产品。如果前缀相同的可推荐产品超过三个，请按字典序返回最小的三个。

请你以二维列表的形式，返回在输入 searchWord 每个字母后相应的推荐产品的列表。

来源：力扣（LeetCode）
链接：https://leetcode.cn/problems/search-suggestions-system
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这道题题意很直观，我提供一个字典树的做法，参考了这个帖子。

代码不难理解，有几个细节提示一下。对于 products 数组里的每个单词，我们都把它 insert 到字典树里。因为之后我们在做 search 的时候需要返回相关的前缀 prefix，所以我们在 insert 的时候，每添加一个字母，我们就把当前这个 product 放入当前这个前缀的 suggestion 里，并一直保持 suggestion 是排序的且长度最大为 3。这样我们就能保证当我们 search 的时候，能返回字典序最小的三个 suggestion。

时间O(mn) - 创建字典树 + insert 每个单词

空间O(n)

Java实现

class Solution {
    public List<List<String>> suggestedProducts(String[] products, String searchWord) {
        Node root = new Node();
        for (String p : products) {
            insert(p, root);
        }
        return search(searchWord, root);
    }

    private void insert(String product, Node root) {
        Node node = root;
        for (int i = 0; i < product.length(); i++) {
            int j = product.charAt(i) - 'a';
            if (node.children[j] == null) {
                node.children[j] = new Node();
            }
            node = node.children[j];
            node.suggestion.offer(product);
            Collections.sort(node.suggestion);
            if (node.suggestion.size() > 3) {
                node.suggestion.pollLast();
            }
        }
    }

    private List<List<String>> search(String searchWord, Node root) {
        List<List<String>> res = new ArrayList<>();
        for (char c : searchWord.toCharArray()) {
            if (root != null) {
                root = root.children[c - 'a'];
            }
            res.add(root == null ? Arrays.asList() : root.suggestion);
        }
        return res;
    }
}

class Node {
    Node[] children = new Node[26];
    LinkedList<String> suggestion = new LinkedList<>();
}

LeetCode 题目总结