0677. Map Sum Pairs

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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"](/kumaeki/LeetCode/wiki/],-["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.

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解法1

用HashMap来保存键值对, 用Tree来实时计算前缀的值.

class MapSum {
    Map<String, Integer> map;
    Node root;
    
    /** Initialize your data structure here. */
    public MapSum() {
        root = new Node(0);
        map = new HashMap<>();
    }
    
    public void insert(String key, int val) {
        
        int value = val;
        // 如果key已经存在, 那么更新tree的值时只更新差值
        if(map.containsKey(key))
            value = val - map.get(key);

        // 更新HashMap的值
        map.put(key, val);

        // 更新Tree中,key所经过的所有节点的值        
        Node node = root;
        for(char c : key.toCharArray()){
            int index = c-'a';
            if(node.next[index] == null)
                node.next[index] = new Node(0);
            
            node.next[index].val += value;
            node = node.next[index];
        }
    }
    
    public int sum(String prefix) {
        Node node = root;
        for(char c : prefix.toCharArray()){
            int index = c - 'a';
            if(node.next[index] == null)
                return 0;
            node = node.next[index];
        }
        return node.val;
    }
}

class Node{
    
    int val;
    Node[] next;
    
    public Node(int val){
        this.val = val;
        next = new Node[26];
    }
}

/**
 * Your MapSum object will be instantiated and called as such:
 * MapSum obj = new MapSum();
 * obj.insert(key,val);
 * int param_2 = obj.sum(prefix);
 */