Algorithms using Java!

Algorithms Topics:

• Big O Concept
• Searching Algorithms
• Sorting Algorithms
• Recursion
• BitWise Operation
• Dynamic Programming

---

Most Basic types of array declaration:

data-type varName[size];

OR

data-type[size] varName;

---

Initializing an array

int[] array = new int[size];

OR

int[] array = {0,1,2,3,4,5,6,7,8,9};

---

Most common methods to find the size or length of an array

arr.length -> for calculating the length of all types of array (int[], String[], char[])

arr.size() -> for calculating the size of an object array(Ex. List array as it stores only objects)

---

Point to remember

.length -> It is used for calculating the length of all types of array (int[], String[], char[])

.length() -> It is used for calculating the length of a String

---

Big O:

Run-time: Time taken or the Iteration made by function/algorithm to process the input. If the number of Iteration are equal to the given input x, then the run time will be O(x).

• O(n): When run-time is proportional to the input size n. In an average case of a linear search algorithm, we have to check each value in an array, the run time in that case is proportional to the size of the array.
  

• O(1): When run-time is constant and independent of the input size.
  

• O(log(n)): When run-time increases exponentially with the size of input. In case of binary search, the worst case is O(log(n)).

---

Searching Algorithms:

Linear Search Algorithm:

    int linearSearch(int[] arr, int searchElement){

        int array[] = arr;
        int sElement = searchElement;

        for(int i=0; i<array.length; i++){
            if(array[i]==sElement)
                return i;
        }

        return -1;
    }

Binary Search Algorithm:

/*--We are assuming that the array is sorted and the array has more than one element--*/    

    int binarySearch(int[] array, int searchElement, int startingIndex, int endingIndex){

        int middleIndex = (startingIndex + endingIndex)/2;


        if(searchElement==array[middleIndex]){
            return middleIndex;
        }


        else if(searchElement<array[middleIndex]){
            return binarySearch(array, searchElement, startingIndex, middleIndex-1);
        }


        else if(searchElement>array[middleIndex])
            return binarySearch(array, searchElement, middleIndex+1, endingIndex);


        else
            return -1;

    }

Time and Space complexities of search algorithms:

Time Complexity:

Search Algorithm	Best Case	Average Case	Worst Case
Linear Search	O(1)	O(n)	O(n)
Binary Search	O(1)	O(logn)	O(logn)

Space Complexity:

Search Algorithm	Space Complexity
Linear Search	O(1)
Binary Search	O(1)

---

Sorting Algorithms:

Bubble Sort:

SWAPS the first two elements of the array and then swaps the adjacent elements, and keeps on SWAPPING until the array is sorted!

Merge Sort:

First DIVIDES the array into two halves, SORT each of them and then MERGE them together!

Quick Sort:

In quick sort, we create a partition known as PIVOT, and then divide the array on the basis of elements less than the PIVOT and the elements larger than the PIVOT element!

Selection Sort:

Performs a LINEAR SCAN, find the smallest element and place it to 0th Index, and then keeps on doing the process until the array is sorted!

Insertion Sort:

In Insertion sort, we COMPARE a element with the elements COMING BEFORE IT in the array, Ex: Element at INDEX=3, will be compared with the elements positioned at INDEX=0,1,2 respectively and then the element is placed at the correct position!

General Sorting Algorithm:

    int[] generalSort(int[] arr){

        int[] ar = {2,3,4,1,8,9,5,6,7};
        int buk = 0;

            for(int i=0; i<ar.length; i++){
                for(int j=i+1; j<ar.length; j++){
                    if(ar[i]>ar[j]) {
                        buk = ar[i];
                        ar[i] = ar[j];
                        ar[j] = buk;
                    }
                }
            }

        return ar;

    }

/*--In built methods for sorting Arrays and Lists--*/

Arrays.sort(stringArray);

Collections.sort(list);

Time and Space complexities of sorting algorithms:

Time Complexities:

Sorting Algorithm	Best Case	Average Case	Worst Case
Bubble Sort	O(n)	O(n^2)	O(n^2)
Merge Sort	O(nlog(n))	O(nlog(n))	O(nlog(n))
Quick Sort	O(nlog(n))	O(nlog(n))	O(n^2)
Selection Sort	O(n^2)	O(n^2)	O(n^2)
Insertion Sort	O(n)	O(n^2)	O(n^2)

Space Complexities:

Sorting Algorithm	Space Complexity(Worst)
Bubble Sort	O(1)
Merge Sort	O(n)
Quick Sort	O(1)
Selection Sort	O(1)
Insertion Sort	O(1)

---

Recursion:

/*--Printing the elements of a SinglyLinkedList in reverse order using recursion--*/


    static void printInReverse(Node head) {

        if(head == null){
        }

        else{
             printInReverse(head.next);
             System.out.println(head.data);
        }

    }

   //--Recursion Stack: [printInReverse1(printInReverse2(printInReverse3(...)))]

   //--Calling Order(LIFO): ..., printInReverse3, printInReverse2, printInReverse1

---

BitWise Operations:

• AND | & : It returns true if both the operands are true.
• OR | | : It returns true if at-least one of the operands is true.
• XOR | ^ : It returns true if exactly one of the operands is true.

---