Added 4Sums readme file and code

0018/4Sums.py

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+class Solution:
+    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
+        ans = set()
+        nums.sort()
+        for i in range(len(nums) - 3):
+            for j in range(i + 1, len(nums) - 2):
+                lo = j + 1
+                hi = len(nums) - 1
+                while lo < hi:
+                    s = nums[i] + nums[j] + nums[lo] + nums[hi]
+                    if s == target:
+                        ans.add((nums[i], nums[j], nums[lo], nums[hi]))
+                        lo += 1
+                    elif s < target:
+                        lo += 1
+                    else:
+                        hi -= 1
+        return list(ans)

0018/README.md

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+# 4Sums
+
+Given an array nums of n integers, return an array of all the unique quadruplets [nums[a], nums[b], nums[c], nums[d]] such that:
+
+0 <= a, b, c, d < n
+a, b, c, and d are distinct.
+nums[a] + nums[b] + nums[c] + nums[d] == target
+You may return the answer in any order.
+
+
+
+## Example 1:
+
+Input: nums = [1,0,-1,0,-2,2], target = 0
+Output: [[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]
+
+## Example 2:
+
+Input: nums = [2,2,2,2,2], target = 8
+Output: [[2,2,2,2]]
+
+
+## Constraints:
+
+1 <= nums.length <= 200
+-109 <= nums[i] <= 109
+-109 <= target <= 109