Antenna Combinations Within Distance

Learning Outcome

Turn the Antenna Combinations Within Distance interview variant into a clear brute-force baseline, optimized pattern, and implementation plan.

Original Interview Statement

Given antenna positions and distance d, count valid pairs or combinations whose difference is less than d.

Examples

Brute Force Approach

Check every pair or every triple directly.

Optimized Approach

Sort positions and use two pointers. For each right, move left until distance is valid, then add combinations from the window.

Exact Pseudocode

sort a
left = 0
for right in range(n):
  while a[right] - a[left] >= d: left += 1
  add right-left valid pairs
  

Reference Code

def count_pairs_within_distance(pos, d):
    pos.sort()
    left = 0
    ans = 0
    for right, value in enumerate(pos):
        while value - pos[left] >= d:
            left += 1
        ans += right - left
    return ans
  

Complexity

Edge Cases

• Equal positions
• Strict < d vs <= d
• Need triples instead of pairs

Follow-ups

• Count triples
• Return actual pairs

Nearest Practice References

• Two pointers counting pairs

Common Mistakes

• Copying the nearest LeetCode solution without checking the changed rule.
• Skipping duplicate or boundary cases.
• Not stating the brute force before the optimized approach.