Advanced · Sorting algorithms · 172 of 264
Sort by frequency
Sort the letters of a word by how often they appear, most frequent first; equally frequent letters in alphabetical order. Keep repeats together.
Input: a word in small letters. Output: the rearranged word, e.g. tree -> eert
Examples
Input
tree
Output
eert
Input
abc
Output
abc
Input
mississippi
Output
iiiissssppm
One more test checks other inputs when you press Check.
Stuck? The idea behind sorting algorithms
Bubble sort swaps neighbours that are in the wrong order, pass after pass. Insertion sort takes each item in turn and slides it back into place among those before it. Merge sort splits the list, sorts each half and merges them.
These exercises ask for the steps, not just the result: write the algorithm yourself rather than calling sorted().
a = [5, 1, 4, 2]
for end in range(len(a) - 1, 0, -1):
for i in range(end):
if a[i] > a[i + 1]:
a[i], a[i + 1] = a[i + 1], a[i]
print(a)Watch for: Off-by-one: comparing a[i] with a[i + 1] means i must stop one before the end.