Implement 4Sum function using kSum helper

4Sum .py

@@ -0,0 +1,34 @@
+def kSum(nums, target, k):
+    res = []
+    if not nums:
+        return res
+    # if smallest sum too large or largest sum too small
+    average = target // k
+    if nums[0] > average or nums[-1] < average:
+        return res
+    if k == 2:
+        # two pointer 2-sum
+        l, r = 0, len(nums)-1
+        while l < r:
+            s = nums[l] + nums[r]
+            if s < target:
+                l += 1
+            elif s > target:
+                r -= 1
+            else:
+                res.append([nums[l], nums[r]])
+                l += 1
+                r -= 1
+                while l < r and nums[l] == nums[l-1]:
+                    l += 1
+        return res
+
+    for i in range(len(nums)):
+        if i == 0 or nums[i] != nums[i-1]:
+            for subset in kSum(nums[i+1:], target - nums[i], k-1):
+                res.append([nums[i]] + subset)
+    return res
+
+def fourSum(nums, target):
+    nums.sort()
+    return kSum(nums, target, 4)