How often every number occurs

MediumMap and Set10 min

Write a function countFrequencies(numbers) that finds the frequency of every number in an array: how many times it occurs.

numbers is an array. Every element is an integer: it can be positive, negative or 0.
Return a Map that works as a counter: the key is the number, the value is how many times it occurs. Keys stay numbers, not strings.
Every number of the array is a key once, with a count of at least 1.
For an empty array return an empty Map.
Key order is not checked.

Examples

countFrequencies([1, 2, 2, 3, 3, 3])

Returns Map(3) { 1 => 1, 2 => 2, 3 => 3 }

countFrequencies([-1, 0, -1])

Returns Map(2) { -1 => 2, 0 => 1 }

countFrequencies([])

Returns Map(0) {}

Asked at ITC Infotech · Source

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InputcountFrequencies([ 1, 2, 2, 3, 3, 3 ])
ExpectedMap(3) { 1 => 1, 2 => 2, 3 => 3 }
InputcountFrequencies([ 0, 7, 0, 4 ])
ExpectedMap(3) { 0 => 2, 7 => 1, 4 => 1 }

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