add arrays left rotation in java (#155)

arrays/README.md

@@ -20,3 +20,4 @@
 
 1. [Counting Inversions](java/count-inversions.java)
 2. [Kadane's Algorithm](java/Kadanes_Algorithm.java)
+2. [Left Rotation of Array](java/left_rotation.java)

arrays/java/left_rotation.java

@@ -0,0 +1,51 @@
+public class left_rotation{
+
+    public static void rotateToLeft(int [] arr){
+        if(arr == null || arr.length == 0){
+            System.out.println("no rotation possible");
+            return;
+        }
+        //take the first element
+        int firstElement = arr[0];
+        //move everything in the left
+        for(int i = 0; i < arr.length -1; i++){
+            arr[i] = arr[i + 1];
+        }
+        //the first element become the last element
+        arr[arr.length - 1] = firstElement;
+    }
+
+    public static void print(int [] arr){
+        System.out.print("[");
+        for(int i = 0; i < arr.length; i++)
+            System.out.print(arr[i] + ", ");
+        System.out.println("]");
+    }
+
+    public static void main(String [] args){
+        int [] arr = {1,2,3,4,5,6,7,8,9};
+        int n = 3;  //number of times to rotate the array
+
+        System.out.print("before: ");
+        print(arr);
+
+        System.out.println("rotating " + n + " times to left");
+        for(int i = 0; i < n; i++)
+            rotateToLeft(arr);  //move to left n times
+
+        System.out.print("after: ");
+        print(arr);
+    }
+}
+
+/*
+    to run the file:
+        javac left_rotation.java
+        java left_rotation
+    
+    result:
+        before: [1, 2, 3, 4, 5, 6, 7, 8, 9, ]
+        rotating 3 times to left
+        after: [4, 5, 6, 7, 8, 9, 1, 2, 3, ]
+        
+*/