Build a frequency map06.01
A frequency map is value → count. One pass: for each x, put x → getOrDefault(x,0)+1. Unseen keys are absent (count 0), not stored as zeros.
This is the map version of a frequency table. Huge or sparse values cannot use an array of size max(a).
worked examples
Same skill, three traces. Read all three before you try it.
01repeats - given
a = [4, 1, 9, 1]- walk
4:1,1:2,9:1- result
{4:1, 1:2, 9:1}
02all same - given
a = [2, 2, 2]- walk
2:3- result
{2:3}
03empty - given
a = []- walk
no keys
- result
{}
Write
Return value → count. Unseen keys are absent, not stored as zeros. Then look up count of
freqMap(a).a is an array of integers.Return value → count. Unseen keys are absent, not stored as zeros. Then look up count of
1 on the first map.a = [4, 1, 9, 1] → {4:1, 1:2, 9:1}.a = [2, 2, 2] → {2:3}.a = [] → {}.constraintsa is a list of integers and may be empty.
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