[Leetcode] Problem 677 - Map Sum Pairs

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Implement a MapSum class with insert, and sum methods.

For the method insert, you’ll be given a pair of (string, integer). The string represents the key and the integer represents the value. If the key already existed, then the original key-value pair will be overridden to the new one.

For the method sum, you’ll be given a string representing the prefix, and you need to return the sum of all the pairs’ value whose key starts with the prefix.

Example

Input: insert(“apple”, 3), Output: Null
Input: sum(“ap”), Output: 3
Input: insert(“app”, 2), Output: Null
Input: sum(“ap”), Output: 5

Code

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public class TrieNode {
    private TrieNode[] children = new TrieNode[128];
    private int weight;
}

public class TrieTree {
    private TrieNode root;
    private Map<String, Integer> sum = new HashMap<>();

    public TrieTree(TrieNode root) {
        this.root = root;
    }

    public void insert(String word, int val) {
        TrieNode node = root;
        int delta = val - sum.getOrDefault(word, 0);

        for (int i = 0; i < word.length(); i++) {
            int idx = word.charAt(i) - 'a';

            if (node.children[idx] == null)
                node.children[idx] = new TrieNode();

            node = node.children[idx];
            node.weight += delta;
        }

        sum.put(word, val);
    }

    public int find(String prefix) {
        TrieNode node = root;

        for (int i = 0; i < prefix.length(); i++) {
            int idx = prefix.charAt(i) - 'a';

            if (node.children[idx] == null)
                return 0;

            node = node.children[idx];
        }

        return node.weight;
    }
}

private TrieTree tree;

public MapSum() {
    tree = new TrieTree(new TrieNode());
}

public void insert(String key, int val) {
    tree.insert(key, val);
}

public int sum(String prefix) {
    return tree.find(prefix);
}