15. Count Positive, Negative, and Zero Values

Difficulty:EasyPattern:Conditional CountingTime:O(n)Space:O(1)

Given an array of integers, count how many values are positive, negative, and zero.

How It Works & Algorithmic Design

Each value belongs to exactly one of three categories:

If value > 0, increment the positive counter.
If value < 0, increment the negative counter.
Otherwise, increment the zero counter.
Return all three counts.

Why This Approach Works Best

A single traversal classifies every element. The algorithm uses $O(n)$ time and $O(1)$ auxiliary space.

Example Walkthrough

Input: [-3, 5, 0, 8, -2, 0]

The array contains all three possible categories.

Counts: Positive=2 | Negative=2 | Zero=2

Each number increments exactly one counter.

Output: { positive: 2, negative: 2, zero: 2 }

The three counters represent the classification.

Answer: Count number categories

function countTypes(arr) {
  let positive = 0;
  let negative = 0;
  let zero = 0;

  for (const value of arr) {
    if (value > 0) positive++;
    else if (value < 0) negative++;
    else zero++;
  }

  return { positive, negative, zero };
}

💡 Use three counters and classify each value using conditional comparisons.

Key Takeaways

Use mutually exclusive conditions when every value belongs to exactly one category.
Three counters are sufficient for this problem.
The solution requires O(n) time and O(1) space.