Turning Recursion Into Iteration

Recursion uses the call stack. An iterative version uses a stack you control. The logic is identical; the difference is that your stack lives on the heap and does not overflow at ten thousand frames.

When you are forced to

• Depth grows with input size and the input can be large.
• You are processing user data of unknown shape - a deeply nested JSON payload will crash a recursive walk.
• You need to pause, resume or cancel partway through.

The simple case: no combining

When the recursion just visits things and does not combine results, the conversion is mechanical - push instead of call.