677. Map Sum Pairs

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LeetCode

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Problem

Design a map that allows you to do the following:

• Maps a string key to a given value.
• Returns the sum of the values that have a key with a prefix equal to a given string.

Implement the MapSum class:

• MapSum() Initializes the MapSum object.
• void insert(String key, int val) Inserts the key-val pair into the map. If the key already existed, the original key-value pair will be overridden to the new one.
• int sum(string prefix) Returns the sum of all the pairs’ value whose key starts with the prefix.

Example 1:

Input
["MapSum", "insert", "sum", "insert", "sum"]
[[], ["apple", 3], ["ap"], ["app", 2], ["ap"]]
Output
[null, null, 3, null, 5]

Explanation
MapSum mapSum = new MapSum();
mapSum.insert("apple", 3);
mapSum.sum("ap");           // return 3 (apple = 3)
mapSum.insert("app", 2);
mapSum.sum("ap");           // return 5 (apple + app = 3 + 2 = 5)

Constraints:

• 1 <= key.length, prefix.length <= 50
• key and prefix consist of only lowercase English letters.
• 1 <= val <= 1000
• At most 50 calls will be made to insert and sum.

Code

208. Implement Trie (Prefix Tree)

class MapSum {
    class TrieNode {
        int sum;
        TrieNode[] children;

        public TrieNode() {
            sum = 0;
            children = new TrieNode[26];
        }
    }

    TrieNode root;
    HashMap<String, Integer> map;
    public MapSum() {
        root = new TrieNode();
        map = new HashMap<>();
    }

    public void insert(String key, int val) {
        int diff = val - map.getOrDefault(key, 0);

        TrieNode node = root;
        for(int i = 0; i < key.length(); i++) {
            int index = key.charAt(i) - 'a';
            if(node.children[index] == null) {
                node.children[index] = new TrieNode();
            }
            node = node.children[index];
            node.sum += diff;
        }

        map.put(key, val);
    }

    public int sum(String prefix) {
        TrieNode node = root;
        for(int i = 0; i < prefix.length(); i++) {
            int index = prefix.charAt(i) - 'a';
            if(node.children[index] == null) return 0;
            node = node.children[index];
        }

        return node.sum;
    }
}