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LeetCode 670 — asked at Meta and Amazon. Given a non-negative integer, swap at most one pair of digits to get the maximum value. Track the last occurrence of each digit, then greedily find the leftmost position where a larger digit appears later — O(n) time, O(1) space.
LeetCode 162 — asked at Google, Meta, and Amazon. Find any peak element in O(log n) using binary search on slope direction. If nums[mid] < nums[mid+1], a peak must exist in the right half — guaranteed by virtual negative infinity at both boundaries.
LeetCode 268 — asked at Amazon, Microsoft, and Google. Find the missing number from 0 to n using the Gauss sum formula in O(n) time and O(1) space. Alternatively use XOR for a bit-manipulation approach. Both are classic array interview questions at FAANG companies.
LeetCode 229 — asked at Amazon, Google, and Microsoft. Find all elements appearing more than n/3 times using the Extended Boyer-Moore Voting Algorithm with two candidates. At most two such elements can exist — verify both candidates with a second pass. O(n) time, O(1) space.
LeetCode 324 — asked at Google and Amazon. Rearrange an array so nums[0] < nums[1] > nums[2] < nums[3]... The key insight: find the median, then use a virtual index mapping to interleave smaller and larger halves without adjacent equal elements. O(n) time with nth_element.