[BE] - Tran Thanh Hai

src/problem4/.gitignore

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+# Dependencies
+node_modules/
+

src/problem4/NOTES.md

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+# Complexity Notes
+
+| Function     | Time | Space |
+|--------------|------|-------|
+| sum_to_n_a   | O(1) | O(1)  |
+| sum_to_n_b   | O(n) | O(1)  |
+| sum_to_n_c   | O(n) | O(n)  |
+
+- **a** — closed-form arithmetic series
+- **b** — iterative accumulation
+- **c** — recursive helper with running total
+
+Assumes `n` yields a result within `Number.MAX_SAFE_INTEGER`.

src/problem4/README.md

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+# Problem 4: Sum to N
+
+Compute the sum of integers from 1 through `n`.
+
+Example: `sum_to_n(5)` → `15`
+
+## Run
+
+```bash
+npm install
+npm start
+```
+
+## Implementations
+
+| Export       | Approach        | Time | Space |
+|--------------|-----------------|------|-------|
+| `sum_to_n_a` | formula         | O(1) | O(1)  |
+| `sum_to_n_b` | while loop      | O(n) | O(1)  |
+| `sum_to_n_c` | recursive helper| O(n) | O(n)  |
+
+See [NOTES.md](./NOTES.md) for brief analysis.