DSA interview questions and answers

Scan once while keeping a running maximum, initializing it to the first element or negative infinity.

Use two pointers at the start and end and swap until they meet.

Track the largest and second-largest while scanning once, updating both when you see a new max and updating second when a value sits between them.

Keep a write index for the next non-zero slot

Walk adjacent pairs and return false on the first decrease.

Reduce k modulo n, reverse the whole array, reverse the first k elements, then reverse the rest.

The expected sum is n*(n+1)/2

Insert every element of both arrays into a hash set, then dump the set into a list.

Two pointers swap from both ends until they meet, which is O(n) time and O(1) extra space.

Compare characters from both ends with two pointers and stop on a mismatch.

If lengths differ they cannot be anagrams.

Count frequencies in a hash map or array, then scan the string again and return the first character with count one.

Trim extra spaces, split on whitespace into words, reverse the word list, then join with single spaces.

Walk the string once and increment a hash map or a 26/128-slot array.

Equal length is required.

While scanning, store each value and its index in a hash map and look up target minus the current number before inserting.

Insert into a hash set and return true as soon as an insert finds the value already present.

Put the smaller array into a hash set, then walk the other and collect values that exist in the set, optionally using a second set for uniqueness.

Boyer-Moore voting keeps a candidate and a count, resetting when the count hits zero

Build a frequency map of the first array, then decrement with the second and reject negatives or leftover keys.