How do you approach Permutations II?

Sort, then use a used[] flag array.

What are common mistakes on Permutations II?

Pushing the working array into the results without copying it, so every entry aliases the same array. Forgetting to undo the choice after the recursive call returns. Deduplicating at the end instead of pruning at the branch - the difference between passing and timing out.

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Permutations II

MediumAlgorithms

Description

Given a collection of numbers nums that might contain duplicates, return all possible unique permutations in lexicographic order.

Example 1

Input: nums = [1,1,2]

Output: [[1,1,2],[1,2,1],[2,1,1]]

Explanation: Three distinct permutations.

Constraints

- 1 <= nums.length <= 8 - -10 <= nums[i] <= 10

Permutations II: approach, complexity and pitfalls

A medium backtracking problem, graded against 6 test cases (4 of them hidden).

The pattern: Backtracking

Systematic exploration of all candidates, abandoning partial ones that cannot work.

Reach for it when you see: "All permutations/combinations/subsets", or constraint puzzles like N-Queens and Sudoku.

If you are stuck

Show the 2-step hint ladder

Sort, then use a used[] flag array.
Skip nums[i] if it equals nums[i-1] and nums[i-1] has not been used in this branch.

The optimal approach

Show the approach: The approach

Backtracking with the standard duplicate rule. Sort `nums`. When choosing the next element, skip `nums[i]` if `nums[i] === nums[i-1]` and `nums[i-1]` is currently unused - this fixes a canonical order among equal elements and prevents duplicate permutations. Iterating over sorted input yields lexicographic output.

Time: O(n·n!). Space: O(n).

The full reference solution in every supported language stays in the editor above - reveal it there once you have had a real attempt.

What the tests check

Read off this problem's own test suite, so these are the cases a submission actually has to survive.

Single element. A one-element case is included. Two-pointer and window solutions frequently return the wrong thing when the two indices start out equal.
Duplicate values. Repeated values appear in the input, so any logic that assumes distinct elements - or that skips over equal neighbours - is exercised.
Negative numbers. The suite includes negative values. Running-sum, min/max seeding and any "initialise to zero" step need to handle them.
Zero. Zero appears in the input, which is the value most likely to break a truthiness check used in place of an explicit comparison.

Common Backtracking mistakes

These apply to the pattern as a whole, not just this problem.

Pushing the working array into the results without copying it, so every entry aliases the same array.
Forgetting to undo the choice after the recursive call returns.
Deduplicating at the end instead of pruning at the branch - the difference between passing and timing out.