Array

LeetCode 448 note: use the array itself as the hash table, marking nums[x - 1] negative for each x; positions still positive are the missing numbers

LeetCode 414 note: track the three largest distinct values, skipping repeats, and return the maximum if fewer than three distinct values exist

LeetCode 350 note: an intersection keeping multiplicity, where Counter’s & takes the smaller count from each side

LeetCode 349 note: convert both arrays to sets and intersect them

LeetCode 283 note: the write pointer advances only on non-zero values, swapping with them, which pushes zeros to the back

LeetCode 268 note: the sum of 0..n minus the array’s sum is the missing number, which may be 0

LeetCode 228 note: scan once, extending the current range when a number continues the previous one and opening a new range otherwise, then format

LeetCode 219 note: a hash map stores each value’s last index; only the most recent occurrence needs a distance check

LeetCode 217 note: if converting to a set shortens the array, there is a duplicate

LeetCode 169 note: Boyer-Moore voting, where different values cancel in pairs and the survivor is the one appearing more than half the time

LeetCode 136 note: x ^ x = 0 and x ^ 0 = x, so XOR everything; pairs cancel and the single number remains

LeetCode 119 note: only row rowIndex is needed; update a 1D array from right to left for O(k) space