Two Pointers & Sliding Window

A lot of array and string problems ship with an obvious answer that reads every pair: two loops, one nested inside the other, quadratic time. Two pointers and sliding window are the two habits that fold that double loop into a single walk across the data. Both keep a couple of indices moving forward and lean on one promise: whatever ground a pointer has already passed, it never has to revisit. That promise is what buys you the drop from O(n squared) to O(n). In a Walmart loop, that drop is the bar.

The two techniques split by how the indices move. Two pointers can converge from opposite ends toward the middle, or run in the same direction at different speeds. A sliding window is a specialized same-direction pair where the gap between the pointers is what you care about. It is a contiguous stretch you widen and narrow to hunt for the best or smallest run that obeys some rule. Name which movement the problem wants, and the skeleton writes itself. The wrinkle the problem wraps around that skeleton is where your interview minutes should go.

The Walmart angle

Walmart's coding bar sits reliably in the medium band, and this cluster is where a nested-loop solution quietly costs you. The editor runs your code against hidden tests, so a brute-force pass that is correct still finishes second to a linear one that reads the same. What the interviewer wants to hear first is the O(n squared) baseline, then the reason two pointers or a window beats it, then the time and space, all before you type. The string-window problems come back often on both the US and India tracks. They arrive in retail terms: the longest run of a feed with no repeated event, the smallest slice of an order log that covers a required set of items. Say the plan out loud, then let the pointers do the work. The narration is half the grade.