Array

LeetCode 3423 note: circular adjacency includes the last and first elements; starting from i = 0 with nums[i - 1] covers it

LeetCode 927 note: every part has the same number of 1s, so count them; the third part always runs to the end of the array, fixing what each part must match, then check the parts do not overlap

LeetCode 2391 note: pickup time is the total garbage count; each truck only drives to the last house with its type, using a prefix sum of travel

LeetCode 2410 note: sort both sides, then same-direction pointers match the weakest player with the smallest trainer that is good enough

LeetCode 3550 note: check indices in increasing order, computing each digit sum; the first match is the answer

LeetCode 1979 note: find the minimum and maximum, then take their GCD with Euclid’s algorithm

LeetCode 2094 note: enumerate the even numbers 100 to 998 instead and check that each digit needed is available; Counter subtraction keeps only positives

LeetCode Problem Solving

LeetCode 952 note: sharing a factor ⇔ sharing a prime factor; factor each number and union indices with the same prime factor using Union-Find, with no pairwise GCDs

LeetCode 1920 note: follow the definition ans[i] = nums[nums[i]]; for O(1) extra space, stack the new value times n on top of the old one

LeetCode 2176 note: n is only 100, so enumerate all pairs; the condition is equal values and the index product i * j divisible by k

LeetCode 164 note: the maximum gap is at least the average gap, so with buckets no wider than that it lies between consecutive non-empty buckets; keep each bucket’s min and max for O(n)