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  2. 2 Additional Online Test (If Required)
  3. 3 Office Interview with Hiring Manager
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  5. 5 Paid Test Task and Follow-Up Interview
  6. 6 Iterative Paid Test Tasks
  7. 7 Office Verification
  8. 8 Final Interview with the Founder

Problems > Pairs With a Given Sum > Editorial

Pairs With a Given Sum — Solution & Editorial

Back to the Problem

Sweep once with a frequency map of earlier elements: each x pairs with the earlier count of K − x.

Complexity: O(N)

Watch out: The pair count can be ~N2/2 ≈ 5×109 — return it in 64 bits.