Combination Sum II
mediumTime: O(2^n)Space: O(n)
Recognize the pattern
combinations summing to target, each element used onceskip duplicate candidatessort first
Brute force idea
Not applicable — enumeration with constraints.
Better approach
Sort. Backtrack from current index (no reuse). Skip candidates[i] when equal to candidates[i-1] at same level. O(2^n).
Key invariant
Sorting groups duplicates. Skipping same-value candidates at the same recursion level prevents duplicate combinations.
Watch out for
Not handling duplicates — [1,1,2] without skipping generates duplicate [1,2] combinations.