two_sum doc

Author: ratulb

docs/index.md

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 ## 🔍 Code Examples
 - [Game of life](game_of_life.md) ➔
  Contains an implementation of [Conway's Game of Life](https://en.wikipedia.org/wiki/Conway%27s_Game_of_Life) cellular automation.
+- [Two sum](two_sum.md) ➔ Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.
+

docs/two_sum.md

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+## Two sum
+
+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.
+ 
+