39. Combination Sum

【Description】
Given a set of candidate numbers (candidates) (without duplicates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target.

The same repeated number may be chosen from candidates unlimited number of times.

Note:

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

Input: candidates = [2,3,6,7], target = 7,
A solution set is:
[
[7],
[2,2,3]
]
Example 2:

Input: candidates = [2,3,5], target = 8,
A solution set is:
[
[2,2,2,2],
[2,3,3],
[3,5]
]

【Idea】

分析题干需求，大致手写列一下解，即知用穷举法，立即推>>DFS

由于return形式是 list[ list[int] ]， 所以在回溯时需要有一个temp list来暂存当前的求解list

【Solution】

class Solution:
    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
        candidates.sort()
        self.resList = []
        self.DFS(candidates, target, 0, [])
        return self.resList
        
    def DFS(self, candis, target, start, value_list):
        if target == 0:    # 存在一组数，和值等于target
            return self.resList.append(value_list)
        length = len(candis)
        for i in range(start, length):
            if target >= candis[i]:     
                self.DFS(candis, target-candis[i], i , value_list + [candis[i]])
            else:   
                return