Count how many times each word appears and store the result in a dictionary.

Split the string on whitespace to get a list of words. Iterate over the list and use a dictionary to keep a running count for each word. In Python you can use collections.Counter which abstracts this logic. Complexity is O(n) time and O(k) space, where n is the number of words and k is the number of unique words. Example:

from collections import Counter
text = "python is easy and python is powerful"
word_counts = Counter(text.split())
print(word_counts)  # Counter({'python': 2, 'is': 2, 'easy': 1, 'and': 1, 'powerful': 1})