DFS，注意能不能重复取，这里能重复取，所以每次都是从头遍历。
class Solution:
def combinationSum(self, candidates, target):
"""
:type candidates: List[int]
:type target: int
:rtype: List[List[int]]
"""
self.reslist=[]
candidates.sort()
self.dfs(candidates, [], target, 0)
return self.reslist
def dfs(self, candidates, sublist, target, last):
if target ==0:
return self.reslist.append(sublist[:])
if target < candidates[0]:
return
for n in candidates:
if target < n:
return
if n < last:
continue
sublist.append(n)
self.dfs(candidates,sublist,target-n,n)
sublist.pop()