41: 2022.04.19 Heap 4th

紀錄/41 2022 0419 Heap 4th.md

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+# LeetCode 讀書會第 41 次聚會 2022/04/19
+
+## leetcode 讀書會通知
+
+1. 項目: 第 41 次聚會
+2. 目的: 線上一起寫題目, 由有想法的人帶領, 先解題, 再看該題有趣的解法
+3. 時間: 04/19 (二) 20:00 ~ 21:00
+4. 地點: google meet 線上 (前 10 分鐘預備鏈接)
+5. 解題項目:  [Heap](https://leetcode.com/explore/featured/card/heap/643/heap/)
+6. 共筆: GitHub https://github.com/programmingbookclub/Leetcode-club
+7. 備註: 
+
+
+
+--- 
+
+* 	[done] MEDIUM	215	Kth Largest Element in an Array	https://leetcode.com/problems/kth-largest-element-in-an-array
+* 	[done] MEDIUM	347	Top K Frequent Elements	https://leetcode.com/problems/top-k-frequent-elements
+* 	EASY	703	Kth Largest Element in a Stream	https://leetcode.com/problems/kth-largest-element-in-a-stream
+* 	EASY	1046	Last Stone Weight	https://leetcode.com/problems/last-stone-weight
+* 	EASY	1337	The K Weakest Rows in a Matrix	https://leetcode.com/problems/the-k-weakest-rows-in-a-matrix
+* 	MEDIUM	378	Kth Smallest Element in a Sorted Matrix	https://leetcode.com/problems/kth-smallest-element-in-a-sorted-matrix
+* 	🔓	MEDIUM	253	Meeting Rooms II	https://leetcode.com/problems/meeting-rooms-ii
+* 	MEDIUM	973	K Closest Points to Origin	https://leetcode.com/problems/k-closest-points-to-origin
+* 	🔓	MEDIUM	1167	Minimum Cost to Connect Sticks	https://leetcode.com/problems/minimum-cost-to-connect-sticks
+* 	MEDIUM	1642	Furthest Building You Can Reach	https://leetcode.com/problems/furthest-building-you-can-reach
+* 	HARD	295	Find Median from Data Stream	https://leetcode.com/problems/find-median-from-data-stream
+
+---
+
+``` python
+import heapq
+class KthLargest:
+    # clarification: 
+    # only add?
+    # upperbound/lowerbound of stream element
+    # 
+    # idea: minheap with fixed size
+    # push the new element into heap, update the heap (if the size is too big), return heap[0]
+    # tc:
+    # O(nlogn) initialize, best case O(n)
+    # O(logn) add, best case O(1)
+    # sc:
+    # O(k) to store elements of stream
+    def __init__(self, k: int, nums: List[int]):
+        self.size = k
+        self.hp = nums
+        heapq.heapify(self.hp)
+        while len(self.hp) > k:
+            heapq.heappop(self.hp)
+
+    def add(self, val: int) -> int:
+        heapq.heappush(self.hp, val)
+        if len(self.hp) > self.size:
+            heapq.heappop(self.hp)
+        return self.hp[0]
+```
+
+``` python
+import heapq
+class Solution:
+    def lastStoneWeight(self, stones: List[int]) -> int:
+        # idea:
+        # use max-heap, (use negative element while inserting into built-in minheap)
+        # put the difference back (even diff == 0) until heap size == 1
+        hp = [-s for s in stones]
+        heapq.heapify(hp)
+
+        while len(hp) != 1:
+            a = heapq.heappop(hp)
+            b = heapq.heappop(hp)
+            heapq.heappush(hp, -1*abs(a-b))
+
+        return -1*hp[0]
+```
+
+``` python
+import heapq
+class Solution:
+    # use maxHeap
+    # calculate the sum of each row and store the (row_sum, index) into max_heap
+    # if the size is too big, pop out the biggest in the heap
+    # if the two biggest has the same sum, pop out the one has bigger index
+    # 
+    # tc: O(H*W) to go through the matrix, where H is height and W is width. O(klogk) to pick out k weakest row index
+    # sc: O(k) to store k weakest rows
+    def kWeakestRows(self, mat: List[List[int]], k: int) -> List[int]:
+        hp = []                                     # store (row_sum, index) in the heap
+        for i, row in enumerate(mat):
+            x = sum(row)
+            heapq.heappush(hp, (-1*x, -1*i))        # both use negative due to built-in minHeap
+            if len(hp) > k:
+                heapq.heappop(hp)
+
+        res = []                # output the answer in order
+        while hp:
+            ppl, index = heapq.heappop(hp)
+            res.append(-1*index)
+
+        return res[::-1]
+```
+
+()