[CountingElements] Codility-FrogRiverOne

Author: Seogeurim

Diff for: README.md

@@ -38,6 +38,7 @@
 | :-: | :-: | :-------------------------------------------------------------------------- | :--- |
 | 01  |     | [Codility-Lesson2 CyclicRotation](./src/Array/CyclicRotation)               |      |
 | 02  |     | [Codility-Lesson2 OddOccurrencesInArray](./src/Array/OddOccurrencesInArray) |      |
+| 03  |     | [Codility-Lesson4 FrogRiverOne](./src/Array/FrogRiverOne)                   |      |
 
 ### Stack
 

Diff for: src/Array/FrogRiverOne/Main.java

@@ -0,0 +1,25 @@
+package Array.FrogRiverOne;
+
+public class Main {
+    public static void main(String[] args) {
+        Solution sol = new Solution();
+        System.out.println(sol.solution(5, new int[]{1, 3, 1, 4, 2, 3, 5, 4}));
+    }
+}
+
+class Solution {
+    public int solution(int X, int[] A) {
+        boolean[] leaves = new boolean[X+1];
+        int count = 0;
+
+        for (int i = 0; i < A.length; i ++) {
+            if (!leaves[A[i]]) {
+                leaves[A[i]] = true;
+                count ++;
+            }
+            if (count == X) return i;
+        }
+
+        return -1;
+    }
+}

Diff for: src/Array/FrogRiverOne/README.md

@@ -0,0 +1,6 @@
+## [Codility - Lesson 4 Counting Elements] FrogRiverOne
+
+> Detected time complexity : **O(N)**
+
+![image](https://user-images.githubusercontent.com/22045163/103459726-86420f80-4d54-11eb-9e07-a5f901f71c04.png)
+![image](https://user-images.githubusercontent.com/22045163/103459780-ce613200-4d54-11eb-9505-ffec8ef16489.png)