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01. Data Structures/05. ArrayList/AddOddAndEvenNums/README.md

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+# Using two ArrayLists, add all even numbers to one list and all odds to another. Do another pass to insert them at the correct indices.
+
+https://leetcode.com/problems/sort-array-by-parity-ii/discuss/194416/java-easy-to-understand-arraylist-solution

01. Data Structures/05. ArrayList/AddOddAndEvenNums/Solution.java

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+import java.util.ArrayList;
+
+class Solution {
+    public int[] sortArrayByParityII(int[] A) {
+        ArrayList<Integer> evens = new ArrayList<Integer>();
+        ArrayList<Integer> odds = new ArrayList<Integer>();
+        for(int i = 0; i < A.length; i++){
+            if(A[i]%2==0)
+                evens.add(A[i]);
+            else
+                odds.add(A[i]);
+        }
+        int index = 0;
+        for(Integer val: evens){
+            A[index] = val;
+            index += 2;
+        }
+        index = 1;
+        for(Integer val: odds){
+            A[index] = val;
+            index += 2;
+        }
+        return A;
+    }
+}

01. Data Structures/05. ArrayList/ArrayList.java

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+package arraylist;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+
+public class Array_List {
+
+	public static void main(String[] args) {
+		// creating ArrayList(AL) of name arr
+
+		ArrayList<Integer> arr1 = new ArrayList<>();
+
+		// 1. add(value) - This will keep on adding value in last of array list.
+		arr1.add(2);
+		arr1.add(7);
+		arr1.add(3);
+		arr1.add(1);
+		arr1.add(7);
+		arr1.add(5);
+		System.out.println(arr1); // [2, 7, 3, 1, 7, 5]
+
+		// 2. size() - return no. of elements added in AL
+		System.out.println(arr1.size());// 6
+
+		// 3. get(index) - return value at particular index
+		System.out.println(arr1.get(2));// 3
+
+		// 4. set(index,value) - change value at particular index
+		arr1.set(2, 8);
+		System.out.println(arr1);// [2, 7, 8, 1, 7, 5]
+
+		// 5. indexOf(value) - return first occurrence index of the particular value if
+		// present, else return -1
+		System.out.println(arr1.indexOf(7)); // 1
+		System.out.println(arr1.indexOf(9)); // -1
+
+		// 6. lastIndexOf(value) - return last occurrence index of the particular value
+		// if present, else return -1
+		System.out.println(arr1.lastIndexOf(7)); // 4
+		System.out.println(arr1.lastIndexOf(9)); // -1
+
+		// 7. remove(index) - remove value at particular index and return the removed
+		// value
+		System.out.println(arr1.remove(2)); // 8
+		System.out.println(arr1); // [2, 7, 1, 7, 5]
+
+		// 8. add(index,value) - add value at particular index. NOTE: Index should lie
+		// in range [0 to arr.size()]
+		System.out.println(arr1); // before : [2, 7, 1, 7, 5]
+		arr1.add(2, 3);
+		System.out.println(arr1); // after : [2, 7, 3, 1, 7, 5]
+
+		// 9. addAll(AL) - appends arr1 into new AL and returns true if successfully
+		// appended
+		ArrayList<Integer> arr2 = new ArrayList<>();
+		arr2.add(777);
+		arr2.add(888);
+		arr2.add(999);
+
+		System.out.println(arr1); // [2, 7, 3, 1, 7, 5]
+		System.out.println(arr2); // Before : [777, 888, 999]
+
+		System.out.println(arr2.addAll(arr1)); // true
+
+		System.out.println(arr2); // After: [777, 888, 999, 2, 7, 3, 1, 7, 5]
+
+		// 10. addAll(index,AL) - appends arr1 into new AL at particular index and
+		// returns true if successfully appended
+		ArrayList<Integer> arr3 = new ArrayList<>();
+		arr3.add(444);
+		arr3.add(555);
+		arr3.add(666);
+
+		System.out.println(arr1); // [2, 7, 3, 1, 7, 5]
+		System.out.println(arr3); // Before : [444, 555, 666]
+
+		System.out.println(arr3.addAll(2, arr1)); // true
+
+		System.out.println(arr3); // After: [444, 555, 2, 7, 3, 1, 7, 5, 666]
+
+		// 11. for loop iteration in ArrayList
+		for (int i = 0; i < arr1.size(); i++) {
+			System.out.print(arr1.get(i) + " "); // 2 7 3 1 7 5
+		}
+		System.out.println(); // new line
+		// for each loop
+		for (int val : arr1) {
+			System.out.print(val + " "); // 2 7 3 1 7 5
+		}
+		System.out.println(); // new line
+
+		// 12. clear() - remove all elements from AL
+		arr1.clear();
+		arr2.clear();
+		arr3.clear();
+		System.out.println(arr1); // []
+		System.out.println(arr2); // []
+		System.out.println(arr3); // []
+
+		// 13. Initialization of new ArrayList with defaultValues.
+		ArrayList<Long> arr4 = new ArrayList<>(Arrays.asList(3l, 4l, 1l, 9l, 56l, 7l, 9l, 12l));
+	}
+}