add initial project files including .gitignore, editor settings, and Two Sum implementation

Easy/TwoSum/TwoSum.md

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+# 1. Two Sum
+### Easy
+
+Hint
+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]`
+
+Explanation: 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.