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class Solution(object):
    def combinationSum2(self, candidates, target):
        """
        :type candidates: List[int]
        :type target: int
        :rtype: List[List[int]]
        """
        self.res = []
        self.cur = []
        candidates = sorted(candidates)
        # Sort candidates first.
        self.backtracking(candidates,0, target)
        return self.res
    def backtracking(self, nums, startIdx, target):
        # Combine elements from startIdx (including) to the end, to sum up to target.
        if target < 0:
            return
        if target == 0:
            self.res.append([each for each in self.cur])
            return
        preNum = -1 # Use preNum to track the previous number.
        for curIdx in range(startIdx,len(nums)):
            curNum = nums[curIdx]
            if curNum == preNum: # If curNum is the same as preNum, e.g. `1` in [1,1,2...].
                continue
            self.cur.append(curNum)
            self.backtracking(nums, curIdx+1, target-curNum)
            # If same element can occur multiple times, backtracking(,curIdx,)
            # if same element can't occur multiple times, backtracking(,curIdx+1,)
            del self.cur[-1]
            preNum = curNum # curNum becomes preNum