Add Standard Deviation (#3039)

src/main/java/com/thealgorithms/maths/StandardDeviation.java

@@ -0,0 +1,22 @@
+package com.thealgorithms.maths;
+
+public class StandardDeviation {
+	
+  public static double stdDev(double[] data)
+	{
+		double var = 0;
+		double avg = 0;
+		for (int i = 0; i < data.length; i++)
+		{
+			avg += data[i];
+		}
+		avg /= data.length;
+		for (int j = 0; j < data.length; j++)
+		{
+			var += Math.pow((data[j] - avg), 2);
+		}
+		var /= data.length;
+		return Math.sqrt(var);
+	}
+  
+}