Trie Node Design and Initialization in DSA Typescript

Mental Model

A trie node holds links to next letters and a marker if a word ends here, like a branching path for words.

Analogy: Imagine a tree where each branch is a letter, and leaves mark the end of a word, so you can follow branches to spell words.

root
 ↓
[ ] -> [ ] -> [ ] -> ... (each box is a child node for a letter)
  ↑
  isEnd=false

Dry Run Walkthrough

Input: Create a trie node for the root with empty children and isEnd false

Goal: Initialize a trie node that can hold up to 26 children and mark if a word ends here

Step 1: Create an array of 26 nulls for children nodes

children: [null, null, null, ..., null] (26 times), isEnd: false

Why: We need space to store links to next letters, one slot per letter

Step 2: Set isEnd flag to false

children: [null, null, null, ..., null], isEnd: false

Why: Initially, no word ends at this node

Result:

children: [null, null, null, ..., null], isEnd: false

Annotated Code

DSA Typescript

class TrieNode {
  children: Array<TrieNode | null>;
  isEnd: boolean;

  constructor() {
    this.children = new Array(26).fill(null); // create 26 slots for letters a-z
    this.isEnd = false; // no word ends here initially
  }
}

// Driver code to create a root node
const root = new TrieNode();
console.log(root.children);
console.log(root.isEnd);

this.children = new Array(26).fill(null);

initialize children array with 26 nulls for each letter slot

this.isEnd = false;

mark node as not ending any word initially

Output

[null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null]

false

Complexity Analysis

Time: O(1) because initialization creates a fixed size array of 26 elements

Space: O(1) for each node since children array size is fixed at 26

vs Alternative: Compared to dynamic structures like maps, fixed array uses constant space and faster access but more memory upfront

Edge Cases

Creating a node with no children

Children array is filled with nulls, no links exist yet

DSA Typescript

this.children = new Array(26).fill(null);

Marking a node as end of word

isEnd flag is set to true when a word finishes at this node

DSA Typescript

this.isEnd = true;

When to Use This Pattern

When you need to store many words sharing prefixes efficiently, use a trie node design with fixed children array and end marker.

Common Mistakes

Mistake: Using a dynamic array or list for children causing slower access
Fix: Use a fixed size array of length 26 for constant time access by letter index

Mistake: Not initializing isEnd flag leading to wrong word end detection
Fix: Always initialize isEnd to false in constructor

Summary

A trie node holds 26 children pointers and a boolean to mark word end.

Use it when building prefix trees to store many words efficiently.

The key is fixed size children array for fast letter lookup and isEnd flag for word completion.