combination-sum.py

# https://leetcode-cn.com/problems/combination-sum/submissions/
# 这题中的元素可以重复使用
class Solution:
    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
        res = []
        track = []
        tracksum = 0
        candidates = sorted(candidates)

        def backtrack(candidates, start, tracksum):
            if tracksum == target:
                res.append(list(track))
            if tracksum > target:
                return
            for i in range(start, len(candidates)):
                if i > 0 and candidates[i] == candidates[i-1]:
                    continue
                track.append(candidates[i])
                tracksum += candidates[i]
                backtrack(candidates, i, tracksum)
                track.pop(-1)
                tracksum -= candidates[i]
        backtrack(candidates, 0, tracksum)
        return res


"""
Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target. You may return the combinations in any order.

The same number may be chosen from candidates an unlimited number of times. Two combinations are unique if the frequency of at least one of the chosen numbers is different.

It is guaranteed that the number of unique combinations that sum up to target is less than 150 combinations for the given input.

 给你一个 无重复元素 的整数数组 candidates 和一个目标整数 target ，找出 candidates 中可以使数字和为目标数 target 的 所有 不同组合 ，并以列表形式返回。你可以按 任意顺序 返回这些组合。

candidates 中的 同一个 数字可以 无限制重复被选取 。如果至少一个数字的被选数量不同，则两种组合是不同的。 

对于给定的输入，保证和为 target 的不同组合数少于 150 个。



Example 1:

Input: candidates = [2,3,6,7], target = 7
Output: [[2,2,3],[7]]
Explanation:
2 and 3 are candidates, and 2 + 2 + 3 = 7. Note that 2 can be used multiple times.
7 is a candidate, and 7 = 7.
These are the only two combinations.
Example 2:

Input: candidates = [2,3,5], target = 8
Output: [[2,2,2,2],[2,3,3],[3,5]]
Example 3:

Input: candidates = [2], target = 1
Output: []

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/combination-sum
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            """