Chains
Intersection of Two Chains
Two paths that merge share a tail. Find the first shared node with a set, or with two pointers that swap starts.
Rebuilding a Number
Two lines of arithmetic - peel the last digit off, append a digit on - and a choice of base. Every problem here is those two lines wearing different clothes.
Lowest Common Ancestor
One question - where do the paths to two nodes split - answered with steadily less information about the tree.
Two Sum
The hash map is the right answer exactly once. Every later question in this family takes away the thing that made it right.