PPractick

The problem

The first letter pattern: row 1 is A, row 2 is AB, row 3 is ABC, and so on. The shape is the familiar right triangle; what's new is producing letters from a position. In C++ a char is a small integer, so 'A' + k is the k-th letter after A. This character arithmetic is used constantly in string problems (frequency arrays, Caesar ciphers, mapping letters to indices), which is why this simple pattern is worth getting exactly right.

Example for n = 4:

A
AB
ABC
ABCD

Function signature

vector<string> letterTriangle(int n);  // row i: the first i letters of the alphabet

Test cases

Input n	Output rows	Explanation
1	"A"
3	"A", "AB", "ABC"	Row i has letters A through the i-th letter
5	"A", "AB", "ABC", "ABCD", "ABCDE"
26	last row is the full alphabet A..Z	Largest valid n

(Confirm: 1 ≤ n ≤ 26; letters are printed without separators.)

How interviewers phrase it

• "Print a triangle of letters: A, AB, ABC..."
• Asked after the number triangle: "Now use letters instead of numbers."

Clarifying questions to ask

1. Uppercase or lowercase?
2. Is N at most 26?
3. Separated by spaces or not?

Follow-ups to expect

1. "How do you get the k-th letter?"
2. "How do you go the other way, from a letter to its index?"
3. "What happens if N is 27?"
4. "What's the complexity?"

Related questions

• Easy and Medium Easy
• Hard Easy
• Pattern 1 Easy
• Pattern 2 Easy
• Pattern 3 Easy
• Pattern 4 Easy

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