Stack

LeetCode 897 note: in-order traversal appending nodes to a right-only chain with left children cleared; keep the dummy separate from the moving tail

LeetCode 844 note: simulate backspaces with a stack; for O(1) space, walk from the end and count how many characters each # skips

LeetCode 682 note: keep valid scores on a stack; C removes the last, D adds its double, and + adds the sum of the last two

LeetCode 590 note: post-order is each child then the node; iteratively, run a root-then-children-right-to-left pre-order and reverse the whole result

LeetCode 589 note: n-ary pre-order records the node then each child in order; flatten recursive results with extend, not by appending a list

LeetCode 234 note: find the middle with fast and slow pointers, reverse the second half, and compare from both ends with O(1) extra space

LeetCode 232 note: two stacks, where in receives pushes and out is refilled from in only when empty, amortized O(1)

LeetCode 225 note: one queue suffices: after a push, rotate all earlier elements to the back so the new element sits at the front

LeetCode 145 note: iterative post-order pushes a node back with a visited flag on its first pop, then the right and left children; record it on the second pop

LeetCode 94 note: iterative in-order traversal, pushing nodes while going left, recording on pop, then turning to the right subtree

LeetCode 144 note: iterative pre-order traversal records on pop and pushes the right child before the left so the left is handled first