Lean4

52. Missing number

EasyProgramFree10 points

xs contains every number from 0 to n exactly once, except one, where n is the length of xs. Write missingNumber xs, the number that is missing.

The tests include long lists, so aim for one pass over the list.

Examples

  1. InputmissingNumber [3, 0, 1]Output2
  2. InputmissingNumber [0, 1]Output2
  3. InputmissingNumber [9, 6, 4, 2, 3, 5, 7, 0, 1]Output8

Submitting also runs 6 hidden tests.

listsarithmeticfolds

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Similar problems

Solution.lean
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3#guard (missingNumber [3, 0, 1]) = (2)
4#guard (missingNumber [0, 1]) = (2)
5#guard (missingNumber [9, 6, 4, 2, 3, 5, 7, 0, 1]) = (8)
examples

Run checks your code against the examples and your custom inputs and records nothing. Submit also runs the hidden tests, confirms the axioms your proof uses and records the result. Each uses one compiler check.

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