added number of islands dsa problem

[agarwalmonika]

This PR adds a C++ solution for the LeetCode Problem 200 – Number of Islands.

Description:
The problem is to count the number of islands in a 2D grid of '1's (land) and '0's (water).
An island is formed by connecting adjacent lands horizontally or vertically.

Approach:

Used Depth-First Search (DFS) to explore connected land cells.

Each time a new land cell is found, DFS marks all its connected cells as visited.

The total count of DFS calls gives the number of islands.

Complexity:

Time: O(M × N) — each cell is visited once.

Space: O(M × N) — recursion stack in the worst case.

Language: C++