DSA interview questions and answers

Keep a prefix-sum frequency map starting with 0:1.

Put all numbers in a set, then only start a streak from values that have no predecessor x-1, and walk x+1, x+2, ...

Hash all sums of A[i]+B[j], then for each C[k]+D[l] add the count of the negated sum.

Same as Two Sum: one pass with value-to-index map, looking up target-x before inserting x so you never reuse the same index.

Sort, then for each index i run two pointers on the right side looking for -nums[i], skipping duplicates at all three positions.

Start at both ends and move the pointer at the shorter line inward, because width shrinks so you must try a taller side.

Walk t with a pointer into s

Squares of a sorted array are largest at the ends.

Grow a right pointer and store last-seen indices in a map

Expand right to grow the sum, then shrink left while the sum stays at least target, tracking the shortest window.

Maintain a window of length p with character counts

Sliding window: the window is valid while window_size minus the count of the most frequent character is at most k.

This is a window where the number of zeros is at most k

Scan left to right and keep a stack of indices with decreasing temperatures

Store values on one stack and running minima on a second stack (or pairs of value and min).

Iterate asteroids and use a stack of survivors.

Use a stack of (previous_string, multiplier) pairs: on '[', push the current buffer and k, on ']', pop and repeat the inner buffer.

Keep a buffer, head index, count (or head and tail), and capacity.

Maintain a frequency map and a queue of candidate characters (or indices).

Multi-source BFS: enqueue every rotten orange at time 0, then spread to adjacent fresh oranges level by level.