思路为将N sum 转换为N-1 Sum, 直到转换为 2 Sum. 不过最开始就已经将nums sort一下, 然后再处理.

def findNsum(nums, target, N, temp, ans):     if len(nums) < N or N < 2 or target < nums[0]*N or target > nums[-1]*N: return     if N == 2:        l, r = 0, len(nums) -1        while l < r:            s = nums[l] + nums[r]            if s == target:                ans.append(temp + [nums[l] , nums[r]])                while l < r and nums[l] == nums[l+1]:                    l += 1                while l < r and nums[r] == nums[r-1]:                    r -= 1                l += 1                r -= 1            elif s < target:                l += 1            else:                r -= 1      else:          for i in range(len(nums) - N + 1):              if i == 0 or nums[i] != nums[i-1]:  # delete the same ans                  findNsum(nums[i+1:], target - nums[i], N -1, temp + [nums[i]], ans)# if want to call it, for example, 4 Sum, ans = []findNsum(sorted(nums), target, 4, [] , ans)return ans