1. Count table
Create one counter for every integer from 0 through maxValue.
const counts = Array(maxValue + 1).fill(0)Typorithm · medium
Counting Sort
▷ Interactive algorithm labcounting-sortsortingdistributioncountingnon-comparison
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Full lesson · 8 learning steps Open any exercise for focused practice
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Step 1 of 8 · intro card
Learn Counting Sort
Sorting Algorithms
Counting Sort counts integer frequencies, then rebuilds the array by walking values in order.
Counting Sort is not comparison-based; it uses values as indexes.
The counts array needs a known, limited value range from 0 through maxValue.
The first pass counts how often each value appears.
The second pass walks the counts array from low to high and emits each value the counted number of times.
It can run in O(n + k), where k is the value range size.
Preparing algorithm lab…
01
Works best for small integer ranges.
02
Use counts[value] to store frequency.
03
Walk the counts array in numeric order.
04
Runtime is O(n + k).
05
This simple version returns a new result array.
function countingSort(arr, maxValue) {
const counts = Array(maxValue + 1).fill(0);
for (const value of arr) {
counts[value]++;
}
const result = [];
for (let value = 0; value < counts.length; value++) {
while (counts[value] > 0) {
result.push(value);
counts[value]--;
}
}
return result;
}Read the implementation
Connect each part of the code to the concept before you type.
2. Count input
Scan the input and count how often each value appears.
for (const value of arr)3. Increment count
The value itself is used as an index into the counts array.
counts[value]++4. Output array
This version writes sorted values into a new result array.
const result = []5. Walk values
Walking counts from low to high produces ascending order.
for (let value = 0; value < counts.length; value++)6. Emit repeats
Repeated values are pushed once for each count.
while (counts[value] > 0)7. Push value
Push the current value into the sorted output.
result.push(value)8. Use one count
Decrease the count after emitting one copy.
counts[value]--A complete learning loop
01
Understand
Read the concept map and follow the code breakdown.
02
Reconstruct
Type the implementation, repair mistakes and work through every recall exercise.
03
Verify
Use the final check without hints; revisit weak concepts in short practice sessions.