The answer
Because it only uses a few discrete variables (max and second_max) to track state, and never allocates new arrays or data structures.
Space Complexity Optimization for Extreme Values
Why is the one-pass method O(1) space?
More FAQs in Space Complexity Optimization for Extreme Values
It means the algorithm requires a fixed, constant amount of extra memory, regardless of how large the input data becomes.
No, converting an array to a Set to remove duplicates requires O(N) extra memory, which is inefficient compared to ignoring duplicates logically with an O(1) variable check.
In real-world systems, RAM is a finite and expensive resource. Algorithms that copy massive datasets unnecessarily can crash servers with OutOfMemory errors.
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