Added a new medium question and solution

Solutions/Solutions/Medium/Medium_099_Number_Of_Islands.swift

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+/*
+https://leetcode.com/problems/number-of-islands/
+#99 Number Of Islands
+Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.
+*/
+import UIKit
+class Medium_099_Number_Of_Islands{
+func numIslands(_ grid: [[Character]]) -> Int {
+    guard grid.count > 0 else { return 0 }
+    guard grid[0].count > 0 else { return 0 }
+    var grid = grid
+    var islandCounter = 0
+    for i in 0..<grid.count{
+        for j in 0..<grid[i].count{
+            if grid[i][j] == "1"{
+                islandCounter += 1
+                LandSink(&grid, i, j)
+            }
+        }
+    }
+    return islandCounter
+}
+func LandSink(_ grid : inout [[Character]], _ i : Int, _ j : Int){
+    if i >= 0 && j >= 0 && i < grid.count && j < grid[i].count && grid[i][j] == "1" {
+        grid[i][j] = "0"
+        LandSink(&grid, i+1, j)
+        LandSink(&grid, i-1, j)
+        LandSink(&grid, i, j+1)
+        LandSink(&grid, i, j-1)
+    }
+    else{
+        return
+    }
+}
+}

Solutions/SolutionsTests/Medium/Medium_099_Number_Of_Islands_Test.swift

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+//
+//  Medium_099_Number_Of_Islands_Test.swift
+//  Medium_099_Number_Of_IslandsTests
+//
+//  Copyright © 2020 Jayant Sogiakar. All rights reserved.
+//
+
+import Foundation
+import XCTest
+@testable import Medium_099_Number_Of_Islands
+class Medium_099_Number_Of_Islands_Test: XCTestCase {
+    func test_001(){
+        let val = numIslands([
+          ["1","1","1","1","0"],
+          ["1","1","0","1","0"],
+          ["1","1","0","0","0"],
+          ["0","0","0","0","0"]
+        ])
+        XCTAssertEqual(val, 1)
+    }
+    func test_002(){
+        let val = numIslands([
+          ["1","1","0","0","0"],
+          ["1","1","0","0","0"],
+          ["0","0","1","0","0"],
+          ["0","0","0","1","1"]
+        ])
+        XCTAssertEqual(val, 3)
+    }
+    func test_3(){
+        let val = numIslands( [["1","0","1","1","0","1","1"]])
+        XCTAssertEqual(val, 3)
+    }
+}