Start with the idea
State the contract before reaching for HashMap: may one index be reused, can values repeat, and what should happen when no pair exists? Then explain the O(n) time and O(n) space tradeoff.
Before coding, clarify whether the function returns values or indices, whether the same index may be used twice, whether duplicate values are allowed, and what 'no solution' means. A Rust signature such as fn two_sum(nums: &[i32], target: i32) -> Option<(usize, usize)> states that the input is borrowed and absence is normal. If arithmetic might overflow, decide whether the input range guarantees safety or whether checked subtraction is needed. This contract prevents an elegant implementation from solving the wrong problem.
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