1. Pick pivot
Use the last value as the pivot for this partition.
const pivot = nums[right]Typorithm · medium
Quick Select
▷ Interactive algorithm labquick-selectdivide-and-conquerpartitionkth-elementone-side
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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 Quick Select
Divide & Conquer
Partition like Quick Sort, but only recurse into the side holding index k — average O(n).
Quick Select finds the kth smallest element without fully sorting the array.
Each partition places the pivot at its final index and splits values around it.
Unlike Quick Sort, you only continue into the half that contains k, halving the work on average.
Preparing algorithm lab…
01
Partition places the pivot exactly.
02
Recurse into one side only.
03
Average O(n), worst O(n^2).
function quickSelect(nums, k) {
function partition(left, right) {
const pivot = nums[right];
let i = left;
for (let j = left; j < right; j += 1) {
if (nums[j] < pivot) {
[nums[i], nums[j]] = [nums[j], nums[i]];
i += 1;
}
}
[nums[i], nums[right]] = [nums[right], nums[i]];
return i;
}
let left = 0;
let right = nums.length - 1;
while (left <= right) {
const pivotIndex = partition(left, right);
if (pivotIndex === k) return nums[pivotIndex];
if (pivotIndex < k) left = pivotIndex + 1;
else right = pivotIndex - 1;
}
return -1;
}Read the implementation
Connect each part of the code to the concept before you type.
2. Move smaller left
Swap values smaller than the pivot to the left region.
[nums[i], nums[j]] = [nums[j], nums[i]]3. Place pivot
Drop the pivot into its final sorted position at index i.
[nums[i], nums[right]] = [nums[right], nums[i]]4. Found k
When the pivot lands at k, it is the kth smallest.
if (pivotIndex === k) return nums[pivotIndex]5. Keep one side
Recurse into only the half that contains k, not both.
if (pivotIndex < k) left = pivotIndex + 1A 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.