Solving Arrays & Strings Leet code questions [Java]

Array is a very basic data structure representing a group of similar elements, accessed by index. Array data structure can be effectively stored inside the computer and provides fast access to the all its elements

Arrays store values of the same data type. Address of elements are stored consecutively in memory
length is fixed. inserting or deleting an element in middle is not easy

Declaration:

int[] nums = {1,2,4} //initializing with values
int[] nums = new int[10] //initializing with size. 0 will be initialized in all indexes if we declare without variables

Accessing an element:

Array index starts from 0..length-1
int[] nums = {1,2,4}
nums[0] — 1
nums[1] — 2
nums[2] — 4

Built in methods to use in Stack

Sorting an element – Arrays.sort(nums);

Finding the size of array – array_name . length

Filling arary element with some values –

Arrays.fill(nums, -1) -> this will assign all values in the array as -1

Searching

When we need to seach a particular element in an array, we can do that in two ways such as

1. Linear search – We need to traverse the array completely and check if we find the element

2. Binary search — We can do binary search only when the array is sorted. Below is the command to use binary search and find if the element is present in array or not.

Arrays.binarySearch(array,value to find) -> returns index

Iterating Arrays

Let us consider we loop the elements in an integer array, we can do it like below

1. For loop:

for (i = 0; i < nums.length; i++) {

System.out.print(nums[i]);   // accessing each element of array

}

2. For each loop

for (int i : nums) {

System.out.print(i);

}

Strings

Strings are nothing but sequence of character values. They will be is written inside “ ”.

Declaration

String s=new String(“Welcome”);

The strings created like this are not mutable. If we want to add or delete anything from this string, we need to use StringBuilder or String Buffer class only

Built-in methods:

length of a string — length()

seeing character at some position — charAt()

if a string contains particular substring — contains()

Converting string to char array — toCharArray()

taking substrings — substring()

Replacing in string — replace()

Replacing first occurence — replaceFirst()

joining two strings — string1.concat(string2)

getting index of a character — string.indexOf(character)

removes beginnind and ending space in string — trim()

converting lower and capital letters inside string — toUpperCase() and toLowerCase()

StringBuilder:

StringBuilder is mutable.

Declaration:

StringBuilder string = new StringBuilder(“Welcome”);

Built in methods used in String Builder:

All string built in methods can be used here as well

string.append(“strings”) — to append to a string

string.insert(startIndex,”string”) — to insert a string from particular index

string.delete(startIndex, endIndex) — deleted string from the start to given end index

string.reverse() — to reverse the array

Iterating String:

For each loop:

for (Char c : string.toCharArray()){

{

System.out.println(c);

}

For loop:

for(int i = 0; i < string.length();i++)

{

System.out.println(string.charAt(i));