40-Combination Sum II

100-Same Tree | Links:

题意:

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] ]

思路

• 回溯法
• 结果需要满足sum条件：用target
• 结果中不能有重复组合：用index控制，每次递归+1
• 每个元素用一次：先排序，遇重复元素跳过

class Solution(object):
    def combinationSum2(self, candidates, target):
        """
        :type candidates: List[int]
        :type target: int
        :rtype: List[List[int]]
        """
        res = []
        if not candidates or target == 0: return res
        candidates.sort()
        
        
        def backtrack(res, temp, nums, target, index):
            if target < 0: return
            if target == 0:
                res.append(temp[:])
                return
            
            for i in range(index, len(nums)):
                if i > index and nums[i] == nums[i-1]: continue
                
                temp.append(nums[i])
                backtrack(res, temp, nums, target - nums[i], i + 1)
                temp.pop()

        backtrack(res, [], candidates, target, 0)
        return res