Q2-6 Majority Element

md/2-06_MajorityElement.md

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+# 2-06 MajorityElement
+Author: WaveAlchemist
+URL: https://leetcode.com/problems/majority-element/description/
+
+Given an array nums of size n, return the majority element.
+The majority element is the element that appears more than ⌊n / 2⌋ times. You may assume that the majority element always exists in the array.
+Example 1:
+Input: nums = [3,2,3]
+Output: 3
+
+Example 2:
+Input: nums = [2,2,1,1,1,2,2]
+Output: 2
+
+# 1st 
+defaultdict nums_countを用いて，各要素の個数をカウント
+各要素の個数が過半数を超えた時点でその要素を返す
+所要時間 4min46s
+実行時間 172ms
+
+``` Python
+class Solution:
+    def majorityElement(self, nums: List[int]) -> int:
+        # dictionary for counting numbers of each element
+        nums_count = defaultdict(int)
+        # count numbers contained in nums
+        for num in nums:
+            if not nums_count[num]:
+                nums_count[num] = 0
+            nums_count[num] += 1
+            if nums_count[num] > (len(nums) // 2):
+                return num
+```
+
+# 2nd
+discord上のほかの議論を参照
+・Boyer-Moore majority vote algorithm
+要素が同じであればcountに+1をし，そうでなければ-1を行う
+要素が過半数ない場合はcountが0をまたぐので，その時点で候補を変更する
+https://leetcode.com/problems/majority-element/solutions/3676530/3-method-s-beats-100-c-java-python-beginner-friendly/
+
+``` Python
+class Solution:
+    def majorityElement(self, nums: List[int]) -> int:
+        count = 0
+        majority_candidate = 0
+        for num in nums:
+            if count == 0:
+                majority_candidate = num
+            if num == majority_candidate:
+                count += 1
+            else:
+                count -= 1
+        return majority_candidate
+```
+
+# 3rd
+・kitakenさんのご指摘
+defaultdictの場合はこの場合分け不要
+
+``` Python
+class Solution:
+    def majorityElement(self, nums: List[int]) -> int:
+        num_count = defaultdict(int)
+        for num in nums:
+            num_count[num] += 1
+            if num_count[num] > len(nums) // 2:
+                return num
+```
+# 4th
+・kouさんのコメント
+こちら len（）がどのくらいコスト掛かるかはみておいても良いと思いました。
+ループの中で使うより先に定義して置く方が良いと思います
+⇒
+timeitを用いて調べると以下のように実行時間に差が出現．
+事前定義をする場合 time: 0.7654874669999856
+事前定義をしない場合 time: 0.9718373919999976
+
+``` Python
+class Solution:
+    def majorityElement(self, nums: List[int]) -> int:
+        nums_count = defaultdict(int)
+        nums_length = len(nums)
+        for num in nums:
+            nums_count[num] += 1
+            if nums_count[num] > nums_length // 2:
+                return num
+```
+

python/2-06_MajorityElement.py

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+
+# %%
+class Solution:
+    def majorityElement(self, nums: List[int]) -> int:
+        # dictionary for counting numbers of each element
+        nums_count = defaultdict(int)
+        # count numbers contained in nums
+        for num in nums:
+            if not nums_count[num]:
+                nums_count[num] = 0
+            nums_count[num] += 1
+            if nums_count[num] > (len(nums) // 2):
+                return num
+
+solution = Solution()
+nums = [1,2,2,2,1]
+print(solution.majorityElement(nums))
+# %%
+import timeit
+from collections import defaultdict
+from typing import List
+
+class Solution1:
+    def majorityElement(self, nums: List[int]) -> int:
+        num_count = defaultdict(int)
+        length_num = len(nums)
+        for num in nums:
+            num_count[num] += 1
+            if num_count[num] > length_num // 2:
+                return num
+
+class Solution2:
+    def majorityElement(self, nums: List[int]) -> int:
+        num_count = defaultdict(int)
+        for num in nums:
+            num_count[num] += 1
+            if num_count[num] > len(nums) // 2:
+                return num
+
+# テストデータを用意
+test_data = [3, 2, 3, 1, 3, 4, 3, 3]
+
+# 計測対象の関数を呼び出すラッパー関数を定義
+def test_solution1():
+    solution = Solution1()
+    return solution.majorityElement(test_data)
+
+def test_solution2():
+    solution = Solution2()
+    return solution.majorityElement(test_data)
+
+# timeitを使用して実行時間を計測
+time_solution1 = timeit.timeit(test_solution1, number=100000)
+time_solution2 = timeit.timeit(test_solution2, number=100000)
+
+print(f"Solution1 time: {time_solution1}")
+print(f"Solution2 time: {time_solution2}")