[LeetCode] Template to N Sum for (LC 15. 3Sum, LC 18. 4Sum)(N>2)

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