1.What does HashMap::insert(k, v) return when the key was already present in the map?
2.Why does the line map["a"] = 1; fail to compile for a Rust HashMap?
3.In *counts.entry(word).or_insert(0) += 1;, what does entry(word).or_insert(0) return?
4.What happens at runtime when you evaluate v[99] on a Vec of length 3?
5.Which C# construct is the closest analog to Rust's map.get(key) returning an Option?
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