Medium

LeetCode 3121 note: track each letter’s last lowercase index and first uppercase index; it counts only when the former comes first

LeetCode 735 note: scan left to right with a stack; only a right-moving top meeting a left-moving asteroid collides, the smaller explodes and equal sizes both do

LeetCode 988 note: DFS prepending each character so the string at a leaf already reads leaf to root, and keep the smallest

LeetCode 1814 note: rearrange the condition into nums[i] - rev(nums[i]) == nums[j] - rev(nums[j]), count pairs per value, and remember the modulus

LeetCode 3075 note: choose k children from the happiest, where the i-th choice loses i, floored at 0

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 3070 note: submatrices containing the top-left corner are exactly the 2D prefix sums P[i][j]; count those <= k

LeetCode 611 note: sort, fix the longest side k, and use two pointers for the shorter sides; when the sum is large enough every i in between works, adding j - i at once

LeetCode 540 note: before the single element every pair starts at an even index, so binary search whether nums[mid] equals nums[mid + 1] at an even mid

LeetCode 275 note: with sorted citations, find the first i where citations[i] >= n - i; the answer is n - i, in O(log n) with binary search

LeetCode 92 note: walk a dummy to the node before left, then move the following nodes one at a time to just after it, right - left times

LeetCode 165 note: split on dots, compare revision by revision as integers, and treat missing revisions of the shorter version as 0