Recursion

LeetCode 25 note: count the length to know how many full groups to reverse, k nodes each; a dummy head avoids special-casing the first group, and the old group head becomes its tail

The choose, recurse, undo template for subsets, permutations, constrained placement, grid search, and pruning

LeetCode 60 note: make k 0-indexed; once the first digit is fixed there are (n-1)! arrangements, so k // (n-1)! picks the first digit among those remaining, then repeat for the next position

LeetCode 394 note: on [ push the current string and count, on ] pop and combine; counts can have several digits

Generating all valid parentheses combinations using backtracking approach

LeetCode 1008 note: splitting the array is O(n^2); recursing with an upper bound is O(n)

LeetCode 2487 note: the kept nodes are non-increasing left to right; keep a monotonic stack, pop anything smaller than the new node, then link what remains

LeetCode 50 note: fast exponentiation, x^n = (x²)^(n/2) with an extra x for odd n; for negative n compute 1 / x^(-n)

LeetCode 143 note: find the middle, reverse the second half, and merge the two alternately, cutting the first half’s tail

LeetCode 24 note: a dummy plus pairs, rewiring prev.next, a.next and b.next together; swap nodes, not values

LeetCode 509 note: the starting point of 1D DP; only the previous two terms are needed, so roll two variables

LeetCode 342 note: confirm a power of two, then that the single 1 sits at an even position: n & 0x55555555 != 0, or n % 3 == 1