Hash-Table

LeetCode 387 note: count first, then scan left to right for the first character with count 1

LeetCode 383 note: every character’s demand in ransomNote must not exceed its supply in magazine; an empty Counter difference means yes

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 290 note: pattern characters and words must correspond one to one; use two dicts or compare set sizes

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

LeetCode 242 note: two strings are anagrams exactly when their character counts match; compare Counters or sorted strings

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 205 note: characters must map one to one; it holds when the distinct characters and distinct pairs all have the same count

LeetCode 202 note: repeatedly take the sum of squared digits, which reaches 1 or enters a cycle; detect it with a set of seen numbers or fast and slow pointers

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