Solution link in github
The problem is a simple one. If you learn Java Strings, it'll be a piece of cake. I suggest you try the problem yourself before proceeding. As a quick reminder, check out this short java string tutorial from tutorialspoint as a guide to this problem.
When taking input, in line 13 I've written an extra sc.nextline(), because when taking a string input before an integer input, omitting line 13 would cause erroneous input. So line 13 works as buffer to correctly take a string input after a numerical input.
The 'frequency' integer array will count the occurrence of each character in the input string. frequency refers to the count of 'a', frequency refers to the count of 'b' and so on. For convenience I've converted the whole string into lowercase. To determine index into this array for a particular character just subtract 'a'. For example index for count of 'c' is 2. So if we encounter 'c' just subtract 'a' from 'c' to get the index 2.
Next we check for the number of elements in the frequency array that have non-zero values. If it's 26 we output 'YES', since all the characters in the alphabet are present. Otherwise print "No".
Hope you find this useful. Check out my Hackerrank Solutions in Java. Don't forget to share and follow :)