feat: add LeetCode problem 1,2

Created new directory for leetcode solutions

Author: nageshnnazare

leetcode/DIRECTORY.md

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+
+# LeetCode
+
+### LeetCode Algorithm
+
+|    # | Title| Solution          | Difficulty |
+| ---- | ------------------------------------------------------------------------------------------------------------------------------------------------------ | ----------------- | ---------- |
+|    1 | [Two Sum](https://leetcode.com/problems/two-sum)| [C++](./src/1.cpp)    | Easy       |
+|    2 | [Add Two Numbers](https://leetcode.com/problems/add-two-numbers)| [C++](./src/2.cpp)    | Medium     |

leetcode/README.md

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+# 📚 Contributing 📚
+
+We're glad you're interested in adding C++ LeetCode solutions to the repository.\
+Here we'll be explaining how to contribute to LeetCode solutions properly.
+
+## 💻 Cloning/setting up the project 💻
+
+First off, you'll need to fork the repository [**here**](https://github.com/TheAlgorithms/C-Plus-Plus/fork).\
+Then, you'll need to clone the repository to your local machine.
+
+```bash
+git clone https://github.com/your-username/C-Plus-Plus.git
+```
+
+After that, you'll need to create a new branch for your solution.
+
+```bash
+git checkout -b solution/your-solution-name
+```
+
+## 📝 Adding a new solution 📝
+
+All LeetCode problems can be found [**here**](https://leetcode.com/problemset/all/).\
+If you have a solution to any of these problems (which are not being **repeated**, that's great! Here are the steps:
+
+1. Add a new file in `leetcode/src` with the number of the problem.
+    - For example: if the problem's number is 98, the filename should be `98.cpp`.
+2. Provide a small description of the solution at the top of the file. A function should go below that. For example:
+
+```c
+/**
+ * Return an vector of vector of size returnSize.
+ * The sizes of the vectors are returned as returnColumnSizes vector.
+ * Note: Both returned vector and columnSizes vector must be dynamically allocated, assume caller calls free.
+ */
+```
+
+3. Do not provide a `main` function. Use the required standalone functions instead.
+4. Doxygen documentation isn't used in LeetCode solutions. Simple/small documentation or comments should be fine.
+5. Don't include libraries/headers such as `iostream`. Your file should be the solution to the problem only.
+
+> **Note**
+> There was a requirement to update the `leetcode/DIRECTORY.md` file with details of the solved problem. It's not required anymore. The information about the problem is fetched automatically throughout the LeetCode API.
+
+## 📦 Committing your changes 📦
+
+Once you're done with adding a new LeetCode solution, it's time we make a pull request.
+
+1. First, stage your changes.
+
+```bash
+git add leetcode/src/98.cpp # Use `git add .` to stage all changes.
+```
+
+2. Then, commit your changes.
+
+```bash
+git commit -m "feat: add LeetCode problem 98" -m "Commit description" # Optional
+```
+
+3. Finally, push your changes to your forked repository.
+
+```bash
+git push origin solution/your-solution-name:solution/your-solution-name
+```
+
+4. You're done now! You just have to make a [**pull request**](https://github.com/TheAlgorithms/C-Plus-Plus/compare). 🎉
+
+If you need any help, don't hesitate to ask and join our [**Discord server**](https://the-algorithms.com/discord)! 🙂

leetcode/src/1.cpp

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+/*
+* 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.
+*/
+
+class Solution {
+public:
+    std::vector<int> twoSum(std::vector<int>& nums, int target) {
+        std::unordered_map<int, int> mp; // value:index
+        std::vector<int> res;
+
+        for (int i = 0; i < nums.size(); i++) {
+            int diff = target - nums[i];
+
+            if (mp.find(diff) != mp.end()) {
+                res.push_back(mp[diff]);
+                res.push_back(i);
+                return res;
+            }
+            mp[nums[i]] = i;
+        }
+        return res;
+    }
+};

leetcode/src/2.cpp

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+/* You are given two non - empty linked lists representing two non - negative integers.
+* The digits are stored in reverse order, and each of their nodes contains a single digit.
+* Add the two numbersand return the sum as a linked list.
+*/
+
+/*
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode() : val(0), next(nullptr) {}
+ *     ListNode(int x) : val(x), next(nullptr) {}
+ *     ListNode(int x, ListNode *next) : val(x), next(next) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
+        int carry = 0;
+        ListNode head;
+        ListNode* sum = &head;
+
+        while (l1 || l2) {
+            int val = carry + (l1 ? l1->val : 0) + (l2 ? l2->val : 0);
+            carry = val / 10;
+            sum->next = new ListNode(val % 10);
+            sum = sum->next;
+            l1 = (l1 ? l1->next : nullptr);
+            l2 = (l2 ? l2->next : nullptr);
+        }
+        if (carry) {
+            sum->next = new ListNode(carry);
+        }
+
+        return head.next;
+    }
+};