解答

  1. use std::collections::HashMap;
  2. use std::hash::Hash;
  4. /// Counter counts the number of times each value of type T has been seen.
  5. struct Counter<T> {
  6.     values: HashMap<T, u64>,
  7. }
  9. impl<T: Eq + Hash> Counter<T> {
  10.     /// Create a new Counter.
  11.     fn new() -> Self {
  12.         Counter { values: HashMap::new() }
  13.     }
  15.     /// Count an occurrence of the given value.
  16.     fn count(&mut self, value: T) {
  17.         *self.values.entry(value).or_default() += 1;
  18.     }
  20.     /// Return the number of times the given value has been seen.
  21.     fn times_seen(&self, value: T) -> u64 {
  22.         self.values.get(&value).copied().unwrap_or_default()
  23.     }
  24. }
  26. fn main() {
  27.     let mut ctr = Counter::new();
  28.     ctr.count(13);
  29.     ctr.count(14);
  30.     ctr.count(16);
  31.     ctr.count(14);
  32.     ctr.count(14);
  33.     ctr.count(11);
  35.     for i in 10..20 {
  36.         println!("saw {} values equal to {}", ctr.times_seen(i), i);
  37.     }
  39.     let mut strctr = Counter::new();
  40.     strctr.count("apple");
  41.     strctr.count("orange");
  42.     strctr.count("apple");
  43.     println!("got {} apples", strctr.times_seen("apple"));
  44. }