The 'no duplicates in the result' constraint is where people slip up.
Clarify the problem constraints (e.g., duplicates, negative numbers, target range) and then propose a backtracking solution that sorts the input, skips duplicates at each level, and explores combinations by moving forward. Discuss time and space complexity, and mention potential optimizations like pruning when the current sum exceeds the target.
Pro tip: Demonstrate awareness of the 'combination sum II' pattern: sorting and skipping duplicates at the same recursion depth is crucial to avoid duplicate combinations. Also, explicitly state that each number can be used only once, so the recursive call must advance the index.
Ask about input size, presence of duplicates, negative numbers, and whether the target can be zero. Confirm that each number can be used at most once and that the output should contain unique combinations.
Explain that you will sort the array to handle duplicates, then use a recursive function that builds combinations, skipping duplicate elements at the same level and pruning when the sum exceeds the target.
Describe the recursive calls: at each step, iterate from the current index, skip if the element equals the previous one (to avoid duplicates), include the element, recurse with the next index, and backtrack. Base case: sum equals target.
State the time complexity (exponential in worst case, e.g., O(2^n) or O(n * 2^n)) and space complexity (O(n) for recursion stack). Mention pruning (break when sum > target) and early termination if the smallest element exceeds the remaining target.
Walk through a small example (e.g., candidates = [10,1,2,7,6,1,5], target = 8) to show how duplicates are skipped and combinations are generated. Discuss edge cases like empty input, no solution, or target smaller than all candidates.
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