The answer
You trade Space for Time. The single loop is incredibly fast (O(N)), but you must consume extra memory (O(N)) to build the Hash Set.
How to Optimize a Nested Loop into a Single Loop
What is the trade-off of this optimization?
More FAQs in How to Optimize a Nested Loop into a Single Loop
The most common way is to eliminate the inner loop by utilizing a Hash Map or Hash Set to store and retrieve data in O(1) constant time.
The complement is the exact value needed to reach the target sum. It is calculated by subtracting the current number from the target (Target - Current).
An inner loop takes O(N) time to scan an array for a value. A Hash Set takes O(1) time to check if it contains a value, making it massively faster.
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