Recursion vs Iteration
Compare recursive and iterative solutions side-by-side. See how they differ in code, complexity, and performance.
Factorial: Recursive vs Iterative
Recursive
int factorialRec(int n) {
if (n <= 1) return 1;
return n * factorialRec(n - 1);
}Time: O(n)Space: O(n) - call stack
Iterative
int factorialIter(int n) {
int result = 1;
for (int i = 2; i <= n; i++) {
result *= i;
}
return result;
}Time: O(n)Space: O(1)
| Aspect | Recursive | Iterative |
|---|---|---|
| Time Complexity | O(n) | O(n) |
| Space Complexity | O(n) - call stack | O(1) |
| Code Clarity | Often more intuitive | More explicit control flow |
| Stack Risk | StackOverflow possible | No stack risk |
| Best For | Tree structures, divide & conquer | Simple loops, large inputs |
Use Recursion When:
- • Problem has natural recursive structure (trees)
- • Code clarity is more important than performance
- • Input size is bounded and small
- • Divide & conquer approach needed
Use Iteration When:
- • Simple linear traversal
- • Performance is critical
- • Very large input sizes expected
- • Memory is constrained
Decision Guide: When to Use Each
| Situation | Recommendation | Why |
|---|---|---|
| Tree/graph traversal | Recursion | Natural tree structure maps to recursive calls |
| Simple counting/looping | Iteration | Simpler, no stack overhead |
| Divide and conquer | Recursion | Problem naturally splits into subproblems |
| Very large input | Iteration | Avoids stack overflow risk |
| Performance critical | Iteration | No function call overhead |
| Code clarity important | Depends | Sometimes recursion is cleaner, sometimes iteration |
Key Differences Summary
Recursion
- ✓Elegant for problems with recursive structure
- ✓Often more intuitive for tree-like problems
- ✓Can express complex logic concisely
- ✗Uses O(n) stack space
- ✗Function call overhead
- ✗Risk of stack overflow
Iteration
- ✓Uses O(1) space (usually)
- ✓No function call overhead
- ✓No stack overflow risk
- ✓Generally faster
- ✗Can be less intuitive for complex problems
- ✗May require explicit stack for tree-like problems
Final Tips
Start with Recursion, Optimize Later
If the problem has recursive structure, write the recursive solution first. It's often easier to get correct. Then optimize to iteration if needed.
Every Recursion Can Be Converted to Iteration
Any recursive algorithm can be written iteratively using an explicit stack. The question is whether the conversion improves clarity or performance.
Know Your Constraints
If input can be very large (n > 10,000), prefer iteration. If you're working with trees or graphs, recursion is often natural and safe.