Is Count Odd Letters from Number easy or hard?

Count Odd Letters from Number is generally classified as an easy problem. The core idea is simple frequency counting, and the optimized solution introduces a common bit manipulation trick for tracking odd and even occurrences.

How to solve Count Odd Letters from Number in O(n)?

Convert the number to a string and iterate through its digits. For each digit, compute a letter index and toggle its bit in an integer mask using XOR. This keeps track of whether the frequency is odd or even. Finally, count the set bits in the mask to get the number of letters with odd frequency.

What is the best approach for Count Odd Letters from Number?

The most efficient approach uses simulation with bit manipulation. Iterate through each digit of the number, map it to a letter index, and toggle the corresponding bit in a bitmask. After processing all digits, count the number of set bits in the mask to determine how many letters appear an odd number of times. This runs in O(n) time and O(1) space.

What data structure is used in Count Odd Letters from Number?

The problem typically uses either a small counting array or a bitmask. The array tracks character frequencies directly, while the bitmask stores parity information using bit manipulation. Both approaches rely on constant-size storage because the alphabet size is fixed.

What is the time complexity of Count Odd Letters from Number?

The optimal solution runs in O(n) time where n is the number of digits in the input number. Each digit is processed once, and the bitmask operations are constant time. Space complexity remains O(1) because the mask stores only a fixed number of bits.

Count Odd Letters from Number Python or Java solution approach?

In Python or Java, convert the number to a string, iterate through each digit, compute a letter index, and toggle the corresponding bit using XOR. After the loop, use a built-in bit count operation to count how many bits are set. The overall complexity is O(n) time and O(1) space.

Is Count Odd Letters from Number asked at Google, Amazon, or Meta?

Problems involving frequency counting, parity tracking, and bitmask toggling appear frequently in interviews at companies like Google, Amazon, and Meta. While this exact problem may vary in wording, the underlying techniques are commonly tested.

Description Hints 1 Solution Video 2:36

3581. Count Odd Letters from Number

Easy85.1% acceptancePremium elsewhere, free here

You are given an integer n perform the following steps:

Convert each digit of n into its lowercase English word (e.g., 4 → "four", 1 → "one").
Concatenate those words in the original digit order to form a string s.

Return the number of distinct characters in s that appear an odd number of times.

Example 1:

Input: n = 41

Output: 5

Explanation:

41 → "fourone"

Characters with odd frequencies: 'f', 'u', 'r', 'n', 'e'. Thus, the answer is 5.

Example 2:

Input: n = 20

Output: 5

Explanation:

20 → "twozero"

Characters with odd frequencies: 't', 'w', 'z', 'e', 'r'. Thus, the answer is 5.

Constraints:

1 <= n <= 10⁹

Topics

Hints open one at a time

Hint 1

Simulate as described

2:36

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