Recursion interview questions

Recursion is a nesting doll that opens to reveal an identical smaller doll inside. In programming, a function solves a substantial problem by delegating smaller versions of the exact same question to fresh invocations of itself. Each invocation operates on shrunken input until hitting an irreducible foundation called a base case, which returns an immediate answer and permits the waiting cascade to resolve backwards.

When to reach for it

Reach for recursion when a problem possesses self-similar subproblems, such as traversing branched tree structures, exploring graph pathways, or generating combinations. Phrases asking for all permutations, subset generation, exhaustive maze navigation, or hierarchical file system traversals signal recursive decomposition. It is natural whenever the answer to a large instance depends on assembling identical solutions for smaller subsets.

How to think about it

Structure every recursive method around two mandatory stages: the termination stop and the shrinking recurrence. Write the base condition first so the function exits before attempting further execution. Next, trust the recursive call to return valid answers for smaller inputs without mentally unwinding every level at once. Pass accumulation state forward through parameters, or combine child return values on the ascent phase once deeper calls return.

What each operation costs