LeetCode.677 Map Sum Pairs （字典序和dfs结合应用）

题目:

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 1:

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

分析：

class TrieNode{
    int times;
    Map<Character,TrieNode> content;
    public TrieNode(){
        content=new HashMap<Character,TrieNode>();
    }
}
public class MapSum {
    //给定单词表，及其出现的次数。
    //当出现新的单词时，更新该单词的次数
    //分别返回由前缀组成的单词个数
    //思路：当遍历到前缀的最后一个字符时，dfs遍历其下面的各次数，因为由该前缀组成的次数便是所求结果

    private TrieNode root;
    /** Initialize your data structure here. */
    public MapSum() {
        root=new TrieNode();
    }
    
    public void insert(String key, int val) {
        //插入
        if(key==null||key.length()==0) return ;
        TrieNode node=root;
        TrieNode tempNode=null;
        
        for(int i=0;i<key.length();i++){
            Character c=key.charAt(i);
            tempNode=node.content.get(c);
            if(tempNode==null){
                //新建节点
                tempNode=new TrieNode();
                //放入node中
                node.content.put(c,tempNode);
            }
            //继续向下
            node=tempNode;
        }
        //已经将单词插入完了，赋值次数
        node.times=val;
        
    }
    
    public int sum(String prefix) {
        //统计由该单词产生的单词总数
        if(prefix==null||prefix.length()==0) return 0;
        
        TrieNode node=root;
        TrieNode tempNode=null;
        for(int i=0;i<prefix.length();i++){
            Character c=prefix.charAt(i);
            tempNode=node.content.get(c);
            if(tempNode==null){
                //说明没有该前缀，直接返回0
                return 0;
            }
            node=tempNode;
        }
        //已经遍历到最后一个字符了，由该HashMap进行dfs遍历
        return dfs(node);
    }
    public int dfs(TrieNode root){
        int count=0;
        for(char c:root.content.keySet()){
            TrieNode temp=root.content.get(c);
            if(temp!=null){
                count+=dfs(temp);
            }
        }
        return count+root.times;
    }
}

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

LeetCode.677 Map Sum Pairs （字典序和dfs结合应用）

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