Nope, that is not correct. Insert is always (amortized) O(1). This is worst time. There is no “bad case”. You only need to be aware that a single insert might

Nope, that is not correct. Insert is always (amortized) O(1). This is worst time. There is no “bad case”. You only need to be aware that a single insert might take longer. But that is not relevant in an absolute majority of cases. “Append to it just slightly too many items” – this “too many” makes sure that you appended enough items so that the expansion amortizes to O(1). That’s the nice thing about a complete mathematical proof – it doesn’t leave any dodgy edge-cases.