DSA interview questions and answers

Place each value v in index v-1 when it lies in 1..n, then scan for the first index i whose value is not i+1.

Track both the max and min product ending at i because a negative can flip min to max.

Treat the array as a functional graph of next = nums[i] and run Floyd cycle detection

Expand right while missing needed counts, then shrink left as long as the window stays valid, tracking the smallest span.

Stack unmatched '(' indices plus a base of the last unmatched ')'

DP where dp[i][j] means s[:i] matches p[:j]

Hash map from key to node plus a doubly linked list of recency: head is most recent, tail is least.

First pass: hash original node to a cloned node with the same value.

Two pointers with left_max and right_max: the water at the shorter side is bounded by that side's max, so you advance the shorter pointer.

Sort, then two nested loops plus an inner two-pointer pair, skipping duplicates at every level.

Maintain a deque of indices with values in decreasing order

Push the current head of each list into a min-heap and track the max among those k heads

Grow a right pointer with a frequency map

Monotonic increasing stack of indices

Scan digits into a number, apply + or - on the running sign, and on '(' push (result, sign) then reset

Two heaps (max-low, min-high) plus delayed deletions via a frequency map, or a policy-balanced multiset.

Greedy: count frequencies, use a max-heap of remaining counts and a cooldown queue of (ready_time, count).

Count k nodes ahead

Push the head of each list into a min-heap of (value, node) and repeatedly pop the smallest, pushing its next.

Bottom-up merge sort: split with slow/fast, merge sorted runs of size 1, 2, 4, ...