Easy

LeetCode 4097 note: only merge the fixed pairs (0,1), (2,3), …, stepping by 2, not every adjacent pair

LeetCode 88 note: merge from the back with O(1) extra space

LeetCode 21 note: the dummy node, and why Python assignment is not a copy

LeetCode 3959 note: accumulate the square sum minus the digit sum; each digit adds d² − d ≥ 0, so the gap never shrinks and you can return as soon as it reaches 50

LeetCode 3950 note: each 1 in n & (n » 1) marks a pair of adjacent ones, so exactly one pair means it has a single 1

LeetCode 3931 note: check that every pair of adjacent digits differs by at most 2

LeetCode note for Best Time To Buy And Sell Stock, rebuilt from the original learning note

LeetCode 3917 note: scan right to left counting odds and evens seen; each position’s answer is the count of opposite parity to its right

LeetCode 3912 note: greater than all on the left means greater than the prefix maximum, and likewise for the right with the suffix maximum; scan both ways and output in original order

LeetCode note for Min Cost Climbing Stairs, rebuilt from the original learning note

LeetCode 3908 note: convert to a string and check that it contains x but does not start with x

LeetCode note for Climbing Stairs, rebuilt from the original learning note