Hash-Table

LeetCode 3071 note: count each value on and off the Y, enumerate the Y value a and background value b (a != b), and take the fewest cells to change

LeetCode 2099 note: pick the k largest values along with their indices, then sort by index to restore the original order

LeetCode 2357 note: each operation zeroes the group of the smallest positive value, so the answer is the number of distinct positive values

LeetCode 264 note: three pointers mark the next ugly numbers to multiply by 2, 3 and 5; take the minimum each time and advance every pointer that produced it

LeetCode 187 note: count every length-10 substring in a hash map and return those seen more than once

LeetCode 142 note: after fast and slow meet, send one back to head and step both one at a time; they meet again at the cycle entry

LeetCode 229 note: at most two values can exceed n / 3; Boyer-Moore keeps two candidates and a second pass verifies their counts

LeetCode 106 note: the last post-order value is the root; taking roots from the end of the post-order, build the right subtree before the left

LeetCode 133 note: during DFS map each old node to its copy and return the existing copy when revisited, avoiding infinite recursion; an empty graph returns None

LeetCode 208 note: each node is a children dict plus an is_end flag; insert, search and startsWith all walk down character by character

LeetCode 138 note: the first pass maps old nodes to new ones, the second wires next and random; seed the dictionary with None -> None

LeetCode 105 note: the first pre-order value is the root, and its position in the in-order tells the left subtree’s size; look positions up in a dictionary for O(n)