Added some more Hard questions 🎉

Author: Jatin-8898

Hard/Calculator.java renamed to Hard/BasicCalculator.java

@@ -1,6 +1,6 @@
 import java.util.*;
 
-class Calculator {
+class BasicCalculator {
     public static void main(String[] args) {
         Scanner s = new Scanner(System.in);
         String str = s.nextLine();

Hard/TrappingRainWater.java

@@ -0,0 +1,45 @@
+class Solution {
+    public int trap(int[] arr) {
+        
+        int n = arr.length;
+        
+        // initialize output 
+		int result = 0; 
+		
+		// maximum element on left and right 
+		int left_max = 0, right_max = 0; 
+		
+		// indices to traverse the array 
+		int lo = 0, hi = n-1; 
+		
+		while(lo <= hi) 
+		{ 
+			if(arr[lo] < arr[hi]) 
+			{ 
+				if(arr[lo] > left_max) 
+	
+				// update max in left 
+				left_max = arr[lo]; 
+				else
+	
+				// water on curr element = 
+				// max - curr 
+				result += left_max - arr[lo]; 
+				lo++; 
+			} 
+			else
+			{ 
+				if(arr[hi] > right_max) 
+				
+				// update right maximum 
+				right_max = arr[hi]; 
+				
+				else
+				result += right_max - arr[hi]; 
+				hi--; 
+			} 
+		} 
+		
+		return result; 
+    }
+}