Heap¶
Table of Contents¶
- 215. Kth Largest Element in an Array (Medium)
- 347. Top K Frequent Elements (Medium)
- 295. Find Median from Data Stream (Hard)
215. Kth Largest Element in an Array¶
import heapq
import random
from typing import List
class Solution:
@staticmethod
def _partition(nums: List[int], left: int, right: int) -> tuple[int, int]:
"""Split a range into values smaller than, equal to, and greater than pivot."""
pivot = nums[(left + right) // 2]
smaller = current = left
greater = right
while current <= greater:
if nums[current] < pivot:
nums[smaller], nums[current] = nums[current], nums[smaller]
smaller += 1
current += 1
elif nums[current] > pivot:
nums[current], nums[greater] = nums[greater], nums[current]
greater -= 1
else:
current += 1
return smaller, greater
def sort(self, nums: List[int], k: int) -> int:
"""Built-in sort: O(n log n) time, O(n) space in Python."""
nums.sort(reverse=True)
return nums[k - 1]
def findKthLargest(self, nums: List[int], k: int) -> int:
"""LeetCode entry point using randomized in-place quickselect."""
return self.quick_select_in_place(nums, k)
def min_heap(self, nums: List[int], k: int) -> int:
"""Min-heap: O(n log k) time, O(k) space."""
min_heap = []
for num in nums:
heapq.heappush(min_heap, num)
if len(min_heap) > k:
heapq.heappop(min_heap)
return min_heap[0]
def quick_sort(self, nums: List[int], k: int) -> int:
"""In-place quicksort: O(n log n) average time, O(log n) average space."""
ranges = [(0, len(nums) - 1)]
while ranges:
left, right = ranges.pop()
if left >= right:
continue
equal_start, equal_end = self._partition(nums, left, right)
left_range = (left, equal_start - 1)
right_range = (equal_end + 1, right)
# Process the smaller range first to keep the stack small.
if left_range[1] - left_range[0] > right_range[1] - right_range[0]:
left_range, right_range = right_range, left_range
ranges.append(right_range)
ranges.append(left_range)
return nums[-k]
def quick_select_recursive(self, nums: List[int], k: int) -> int:
"""
Find the kth largest using recursive list partitions.
k largest
3, 2, 1, 5, 6, 4
0 1 2 3 4 5
Time: O(n) average, O(n^2) worst case.
Space: O(n) average for partitions and recursion.
"""
pivot = random.choice(nums)
left = [number for number in nums if number > pivot]
mid = [number for number in nums if number == pivot]
right = [number for number in nums if number < pivot]
if len(left) >= k:
return self.quick_select_recursive(left, k)
if len(left) + len(mid) >= k:
return pivot
return self.quick_select_recursive(right, k - len(left) - len(mid))
def quick_select_in_place(self, nums: List[int], k: int) -> int:
"""
Find the kth largest using randomized in-place quickselect.
The kth largest is the (n - k)th smallest using a zero-based index.
Time: O(n) average, O(n^2) worst case.
Space: O(1), because partitioning is iterative and in place.
"""
target = len(nums) - k
def partition(left: int, right: int) -> int:
pivot = random.randint(left, right)
nums[pivot], nums[right] = nums[right], nums[pivot]
pivot = nums[right]
store = left
for index in range(left, right):
if nums[index] < pivot:
nums[index], nums[store] = nums[store], nums[index]
store += 1
nums[store], nums[right] = nums[right], nums[store]
return store
left, right = 0, len(nums) - 1
while left <= right:
pivot_index = partition(left, right)
if pivot_index == target:
return nums[pivot_index]
if pivot_index < target:
left = pivot_index + 1
else:
right = pivot_index - 1
raise RuntimeError("quickselect failed to find the target")
if __name__ == "__main__":
solution = Solution()
methods = (
solution.findKthLargest,
solution.sort,
solution.min_heap,
solution.quick_sort,
solution.quick_select_recursive,
solution.quick_select_in_place,
)
cases = (
([3, 2, 1, 5, 6, 4], 2, 5),
([3, 2, 3, 1, 2, 4, 5, 5, 6], 4, 4),
([-1, -1], 2, -1),
([1], 1, 1),
)
for method in methods:
for nums, k, expected in cases:
assert method(nums.copy(), k) == expected
347. Top K Frequent Elements¶
-
Tags: Array, Hash Table, Divide And Conquer, Sorting, Heap Priority Queue, Bucket Sort, Counting, Quickselect
import heapq
from collections import Counter
from typing import List
# Heap + Counter
def topKFrequent(nums: List[int], k: int) -> List[int]:
minHeap = []
for val, freq in Counter(nums).items():
if len(minHeap) < k:
heapq.heappush(minHeap, (freq, val))
else:
heapq.heappushpop(minHeap, (freq, val))
return [i for _, i in minHeap]
# Counter (Most Common)
def topKFrequentCounter(nums: List[int], k: int) -> List[int]:
commons = Counter(nums).most_common(k)
return [i for i, _ in commons]
nums = [1, 1, 1, 2, 2, 3]
k = 2
print(topKFrequent(nums, k)) # [1, 2]
print(topKFrequentCounter(nums, k)) # [1, 2]
295. Find Median from Data Stream¶
from heapq import heappop, heappush
class MedianFinder:
def __init__(self):
self.minHeap = []
self.maxHeap = []
self.min_size = 0
self.max_size = 0
def addNum(self, num: int) -> None:
heappush(self.maxHeap, -num)
heappush(self.minHeap, -heappop(self.maxHeap))
self.min_size += 1
if self.min_size > self.max_size:
heappush(self.maxHeap, -heappop(self.minHeap))
self.min_size -= 1
self.max_size += 1
def findMedian(self) -> float:
if self.min_size == self.max_size:
return (-self.maxHeap[0] + self.minHeap[0]) / 2.0
return float(-self.maxHeap[0])
def test_median_finder():
obj = MedianFinder()
obj.addNum(1)
obj.addNum(2)
assert obj.findMedian() == 1.5
obj.addNum(3)
assert obj.findMedian() == 2.0
obj.addNum(4)
assert obj.findMedian() == 2.5
obj.addNum(5)
assert obj.findMedian() == 3.0
#include <iostream>
#include <queue>
#include <vector>
using namespace std;
class MedianFinder {
private:
priority_queue<int> maxHeap;
priority_queue<int, vector<int>, greater<int>> minHeap;
int max_size = 0;
int min_size = 0;
public:
MedianFinder() {}
void addNum(int num) {
if (min_size == max_size) {
minHeap.push(num);
maxHeap.push(minHeap.top());
minHeap.pop();
max_size++;
} else {
maxHeap.push(num);
minHeap.push(maxHeap.top());
maxHeap.pop();
min_size++;
}
}
double findMedian() {
if (min_size == max_size) {
return (maxHeap.top() + minHeap.top()) / 2.0;
} else {
return (double)maxHeap.top();
}
}
};
int main() {
MedianFinder* obj = new MedianFinder();
obj->addNum(1);
obj->addNum(2);
cout << obj->findMedian() << endl; // 1.5
obj->addNum(3);
cout << obj->findMedian() << endl; // 2
return 0;
}