# Leetcode 40 - Combination sum II

Note:

• This is a backtracking problem
• The key is to remove repetitive elements that have appeared on the same level
• Check i > startIndex because the leftmost node on each level can have the same value as its parent.
• For example [1, 1, 2]. To avoid [1,2] appearing twice, we have to check if candidates[i] == candidates[i-1] in the for loop.
• Rememebr to sort candidates first before iterating, so elements with same value will be neighbors.

Given a collection of candidate numbers (candidates) and a target number (target), find all unique combinations in candidates where the candidate numbers sum to target.

Each number in candidates may only be used once in the combination.

Note: The solution set must not contain duplicate combinations.

Example