Given a string $S,$ output the count of number of substrings of length four such that

every substring should contain unique characters in it$.$ If such substring exists,

output $0.$

Given an array of integers, output all the pairs of integers whose sum is equal to $60$

$(2$ Sum problem in Leetcode $)$

Reverse the string without using extra variable and built in functions

Delete $d$ digits from a n digit number $(dn),$ so that resulting number is minimum

$($ Remove K digits problem in Leetcode $)$give answer to the three qns in the image pls