feat(1-two-sum): add README

0001_two-sum/README.md

@@ -0,0 +1,36 @@
+Given an array of integers `nums` and an integer `target`, return *indices of the two numbers such that they add up to `target`*.
+
+You may assume that each input would have *exactly* one solution, and you may not use the *same* element twice.
+
+You can return the answer in any order.
+
+Example 1:
+
+```
+Input: nums = [2,7,11,15], target = 9
+Output: [0,1]
+Output: Because nums[0] + nums[1] == 9, we return [0, 1].
+```
+
+Example 2:
+
+```
+Input: nums = [3,2,4], target = 6
+Output: [1,2]
+```
+
+Example 3:
+
+```
+Input: nums = [3,3], target = 6
+Output: [0,1]
+```
+
+Constraints:
+
+-   `2 <= nums.length <= 104`
+-   `-109 <= nums[i] <= 109`
+-   `-109 <= target <= 109`
+-   Only one valid answer exists.
+
+Follow-up: Can you come up with an algorithm that is less than `O(n2) `time complexity?