40. Combination Sum II

一个正整数的列表candidates,中间的数字是无序的且有可能有重复,现给定一个正整数的目标target,从candidates中取出若干个数使其和为target,试求所有中可能(重复的不算)

"""
40. Combination Sum II
Medium

619

32

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

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

Note:

All numbers (including target) will be positive integers.
The solution set must not contain duplicate combinations.
Example 1:

Input: candidates = [10,1,2,7,6,1,5], target = 8,
A solution set is:
[
  [1, 7],
  [1, 2, 5],
  [2, 6],
  [1, 1, 6]
]
Example 2:

Input: candidates = [2,5,2,1,2], target = 5,
A solution set is:
[
  [1,2,2],
  [5]
]

拿到这道题目的思路:

首先要给candidates排个序,

将结果分为两类带最后一个数字的和不带最后一个数字的,

但是这样有一个问题就是说如果最后两个数字相同有可能会有重复:解决办法:1,求完以后去重2,事先就去重

我用的第2种方法,假定最后一个数字相同的一定有k个

那么不带最后一个数字的组合中,最后一个数字个数在k-1以内的也包含了也就是重复部分,

我们在此基础上添加一共含有k个最后数字的组合即可

class Solution:
    def combinationSum2(self, candidates, target):
        """
        :type candidates: List[int]
        :type target: int
        :rtype: List[List[int]]
        """
        if not candidates:
            return []
        candidates.sort()
        lenc = len(candidates)
        if lenc == 1:
            if target == candidates[0]:
                return [[target]]
        result = self.combinationSum2(candidates[0:-1], target)
        k, lastnum = 0, candidates[-1]
        for i in range(lenc-1, -1, -1):
            if candidates[i] == lastnum:
                k += 1
            else:
                break
        targetot = target - lastnum*k
        if targetot == 0:
            result.append([lastnum]*k)
            return result
        elif targetot < 0:
            return result
        else:
            resultot = self.combinationSum2(candidates[:lenc-k], targetot)
            for i in resultot:
                result.append(i+[lastnum]*k)
        return result

这里考虑到一个问题就是排序只要开始排一下就行了,改进一下

class Solution:
    def sortedCombinationSum2(self, candidates, target):
        if not candidates:
            return []
        lenc = len(candidates)
        if lenc == 1:
            if target == candidates[0]:
                return [[target]]
        result = self.sortedCombinationSum2(candidates[0:-1], target)
        k, lastnum = 0, candidates[-1]
        for i in range(lenc-1, -1, -1):
            if candidates[i] == lastnum:
                k += 1
            else:
                break
        targetot = target - lastnum*k
        if targetot == 0:
            result.append([lastnum]*k)
            return result
        elif targetot < 0:
            return result
        else:
            resultot = self.sortedCombinationSum2(candidates[:lenc-k], targetot)
            for i in resultot:
                result.append(i+[lastnum]*k)
        return result
    
    def combinationSum2(self, candidates, target):
        """
        :type candidates: List[int]
        :type target: int
        :rtype: List[List[int]]
        """
        candidates.sort()
        return self.sortedCombinationSum2(candidates, target)