基礎構文とデータ構造

1. 変数と基本操作

Two Sum（二数の和）

# アプローチ1：全探索
class Solution:
    def twoSum(self, nums, target):
        for i in range(len(nums)):
            for j in range(i + 1, len(nums)):
                if nums[i] + nums[j] == target:
                    return [i, j]

# アプローチ2：ハッシュマップ
class Solution:
    def twoSum(self, nums, target):
        seen = {}
        for idx, val in enumerate(nums):
            complement = target - val
            if complement in seen:
                return [seen[complement], idx]
            seen[val] = idx

Reverse Integer（整数の反転）

# 文字列スライス方式
class Solution:
    def reverse(self, x):
        sign = -1 if x < 0 else 1
        rev = int(str(abs(x))[::-1]) * sign
        return rev if -(2**31) <= rev < 2**31 else 0

# 数学的アプローチ
class Solution:
    def reverse(self, x):
        sign = -1 if x < 0 else 1
        x = abs(x)
        result = 0
        while x:
            result = result * 10 + x % 10
            x //= 10
        rev = result * sign
        return rev if -(2**31) <= rev < 2**31 else 0

Palindrome Number（回文数判定）

class Solution:
    def isPalindrome(self, x):
        return str(x) == str(x)[::-1]

2. 条件分岐とループ

Roman to Integer（ローマ数字を整数に変換）

class Solution:
    def romanToInt(self, s):
        mapping = {
            'I': 1, 'V': 5, 'X': 10, 'L': 50,
            'C': 100, 'D': 500, 'M': 1000,
            'IV': 4, 'IX': 9, 'XL': 40,
            'XC': 90, 'CD': 400, 'CM': 900
        }
        total = 0
        i = 0
        while i < len(s):
            if i + 1 < len(s) and s[i:i+2] in mapping:
                total += mapping[s[i:i+2]]
                i += 2
            else:
                total += mapping[s[i]]
                i += 1
        return total

Longest Common Prefix（最長共通接頭辞）

# zipを使った方法
class Solution:
    def longestCommonPrefix(self, strs):
        prefix = ""
        for chars in zip(*strs):
            if len(set(chars)) == 1:
                prefix += chars[0]
            else:
                break
        return prefix

# 縮小比較方式
class Solution:
    def longestCommonPrefix(self, strs):
        if not strs:
            return ""
        candidate = strs[0]
        for s in strs[1:]:
            while not s.startswith(candidate):
                candidate = candidate[:-1]
                if not candidate:
                    return ""
        return candidate

# ソート後比較方式
class Solution:
    def longestCommonPrefix(self, strs):
        if not strs:
            return ""
        strs.sort()
        first, last = strs[0], strs[-1]
        i = 0
        while i < len(first) and i < len(last) and first[i] == last[i]:
            i += 1
        return first[:i]

Valid Parentheses（括弧の妥当性検証）

class Solution:
    def isValid(self, s):
        mapping = {')': '(', ']': '[', '}': '{'}
        stack = []
        for char in s:
            if char in mapping:
                top = stack.pop() if stack else '#'
                if mapping[char] != top:
                    return False
            else:
                stack.append(char)
        return not stack

3. リストと文字列操作

Remove Duplicates from Sorted Array（重複要素の削除）

class Solution:
    def removeDuplicates(self, nums):
        if not nums:
            return 0
        write_index = 1
        for read_index in range(1, len(nums)):
            if nums[read_index] != nums[read_index - 1]:
                nums[write_index] = nums[read_index]
                write_index += 1
        return write_index

Length of Last Word（末尾単語の長さ）

class Solution:
    def lengthOfLastWord(self, s):
        end = len(s) - 1
        while end >= 0 and s[end] == ' ':
            end -= 1
        start = end
        while start >= 0 and s[start] != ' ':
            start -= 1
        return end - start

関数とアルゴリズム基礎

1. 関数と再帰

• Merge Two Sorted Lists
• Climbing Stairs
• Fibonacci Number

2. ソートと探索

• Merge Sorted Array
• Search Insert Position
• First Bad Version

3. 二重ポインタとスライディングウィンドウ

• Move Zeroes
• Reverse String
• Valid Palindrome

データ構造の応用

1. スタックとキュー

• Min Stack
• Implement Queue using Stacks
• Implement Stack using Queues

2. ハッシュテーブルとセット

• Contains Duplicate
• Intersection of Two Arrays
• First Unique Character in a String

3. リンクリスト

• Reverse Linked List
• Linked List Cycle
• Intersection of Two Linked Lists

アルゴリズム思考の深化

1. 貪欲法

• Best Time to Buy and Sell Stock
• Assign Cookies
• Can Place Flowers

2. 分割統治と再帰

• Majority Element
• Maximum Subarray
• Pow(x, n)

3. 動的計画法

• House Robber
• Min Cost Climbing Stairs
• Unique Paths

総合問題

• Container With Most Water
• 3Sum
• Group Anagrams
• Binary Tree Inorder Traversal
• Symmetric Tree