The answer
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).
How to Optimize a Nested Loop into a Single Loop
What is the complement in the Two Sum problem?
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.
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.
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.
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