Recursion, Sorting and Searching

Recursion, sorting, and searching are the bread and butter of a coding round. An interviewer who hands you “merge two sorted lists” or “find the missing number” is checking whether you can pick the right algorithm and state its cost without hesitation. This lesson builds the core tools from scratch: recursion (base case, recursive case, and the call stack), merge sort and quicksort, and linear versus binary search. Then it works three of our interview questions end to end. Sorting Without Sort() rebuilds merge sort, Finding the Largest Possible Number sorts with a custom comparator, and Find the Missing Number applies binary search in $O(\log n)$.