A trie is a tree-like data structure in which every node stores a character. A tire can store one or multiple words. After building a trie, strings or substrings can be retrieved by traversing down a path (a branch) of the trie. Tries are used to find substrings, autocomplete and many other string operations. In this post, the data structure to store child nodes is a hash map.

trie diagram

Why is a hash map used in a trie node?

In a trie, each trie node might have multiple branches. So a data structure is used to point to next nodes in branches. The data structure can be an array, a linked list or a hash map. When the data structure is a hash map, the next character stored in the current node is used as the key to find the node to hold the next character in the branch. Using a hash map is the best of three implementations since its put and get operations take O(1) time.


Table of Content


Map of trie implementations

Part 1 – trie implementation using array
Part 2 – trie implementation using linked list

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Part 3 – trie implementation using hash map


Define classes in trie implementation

Like building a tree, you need to define a TrieNode class before a Trie class. A TrieNode class has three variables: data stores a character. children is a data structure that points to child nodes (branches). Here the data structure is hash map. isEnd is to mark whether this is the last node in the branch. In Trie class, there is one variable root. The operation of insertion, search or deletion always starts from root.

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Insert a word in trie implementation

To insert a word, first check whether the word is in the trie so that you don’t insert a duplicate word. Then loop through each character in the word. Create a node to store the next character. Use the next character as the key and the new node as the value to create an entry. Store the new entry in the current node’s children. Then move to the child node. When the node is at the last character of the word, you mark the node‘s isEnd to be true.

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Doodle

trie insert array


Delete a word

To delete a word from a trie, first check whether the word exists in the trie. If not, the method can return. The same as insertion, loop through each character in the word. A pointer node starts from root. If the character is not in the node‘s children, the method returns directly. Otherwise, the node moves to the child node. When the node is at the last character of the word, mark the node‘s isEnd to be false.

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To search a word in a trie, loop through each character in the word. A pointer node starts from root. If the character is not in the node‘s children, the method returns false. Otherwise, the node moves to the child node. When the node is at the last character of the word, return the node‘s isEnd value, which can be true or false (deleted).

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Print all words in trie

To print all words in a trie, recursion is used to traverse all nodes in the trie. This is similar to preorder (DFS, depth first search) of a tree. When visiting the node, the method concatenates characters from previously visited nodes with the character of the current node. When the node‘s isEnd is true, the recursion reaches the last character of the word, and add the word to the result list.

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