About the usefulness of the Python Counter class-You don't have to count it yourself anymore-

Overview

Until now, in order to check how many elements are included in the list, the list element was rotated by the for statement and the combination of the element and the number was set in the dict. However, if you use the collections.Counter class, you can process it in one line.

So far (dirty with dict)

    a = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5]
    b = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 6, 6, 6, 6, 6, 6]

    a_dict = dict()
    b_dict = dict()

    for item in a:
        if item in a_dict:
            a_dict[item] += 1
        else:
            a_dict[item] = 1

    for item in b:
        if item in b_dict:
            b_dict[item] += 1
        else:
            b_dict[item] = 1

    print(a_dict) #{1: 1, 2: 2, 3: 3, 4: 4, 5: 5}
    print(b_dict) #{1: 1, 2: 2, 3: 3, 4: 4, 6: 6}

With the Counter class

    from collections import Counter

    a = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5]
    b = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 6, 6, 6, 6, 6, 6]

    a_counter = Counter(a)
    b_counter = Counter(b)
    print(a_counter) #Counter({5: 5, 4: 4, 3: 3, 2: 2, 1: 1})
    print(b_counter) #Counter({6: 6, 4: 4, 3: 3, 2: 2, 1: 1})

In the Counter class, union and difference sets can be calculated.

    from collections import Counter

    a = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5]
    b = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 6, 6, 6, 6, 6, 6]

    a_counter = Counter(a)
    b_counter = Counter(b)

    print(a_counter + b_counter) #Counter({4: 8, 3: 6, 6: 6, 5: 5, 2: 4, 1: 2})
    print(a_counter - b_counter) #Counter({5: 5})

Problems that can be easily solved by using the Counter class in AtCoder

• https://atcoder.jp/contests/abc181/tasks/abc181_d → I learned about the existence of Counter from this explanation
• https://atcoder.jp/contests/abc091/tasks/abc091_b