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Maps And Sets

The most commonly used pieces of code related to the topic in competitive programming.

Counting with a dictionary​

Merge​

Even if there is no key, merging can be used:

    Map<String, Integer> dict = new HashMap<>();
    for (int c = 0; c < str.length(); i++) {
        String s = String.valueOf(str.charAt(c));
        dict.merge(s, 1, Integer::sum);
    }

AtomicInteger​

Not quite the fastest way:

    Map<Character, AtomicInteger> dict = new HashMap<>();
    for (int c = 0; c < str.length(); c++) {
        dict.computeIfAbsent(str.charAt(c), k-> new AtomicInteger(0)).incrementAndGet();
    }

Sum up integer dictionary values​

Quickly count the values, with additional filtering can apply in the stream:

    Map<String, Integer> dict = new HashMap<>();
    //...
    dict.values().stream().mapToInt(Integer::intValue).sum();

Printing a set​

Simply with enhanced 'for', see JSR 201:

    for (String s : set) {
        writer.write(s);
    }

Printing a map​

With enhanced 'for':

    for (Map.Entry<String, Object> entry : map.entrySet()) {
        writer.write(entry.getKey() + ":" + entry.getValue().toString());
    }