유효한 괄호

20. Valid Parentheses

https://leetcode.com/problems/valid-parentheses/

Write a function that reverses a string. The input string is given as an array of characters s.

Example 1:

Input: s = ["h","e","l","l","o"] Output: ["o","l","l","e","h"]

Example 2:

Input: s = ["H","a","n","n","a","h"] Output: ["h","a","n","n","a","H"]

Constraints:

• 1 <= s.length <= 105
• s[i] is a printable ascii character.

Follow up: Do not allocate extra space for another array. You must do this by modifying the input array in-place with O(1) extra memory.You are given an array of logs. Each log is a space-delimited string of words, where the first word is the identifier.

There are two types of logs:

• Letter-logs: All words (except the identifier) consist of lowercase English letters.
• Digit-logs: All words (except the identifier) consist of digits.

Reorder these logs so that:

1. The letter-logs come before all digit-logs.
2. The letter-logs are sorted lexicographically by their contents. If their contents are the same, then sort them lexicographically by their identifiers.
3. The digit-logs maintain their relative ordering.

Return the final order of the logs.

Example 1:

Input: logs = ["dig1 8 1 5 1","let1 art can","dig2 3 6","let2 own kit dig","let3 art zero"] Output: ["let1 art can","let3 art zero","let2 own kit dig","dig1 8 1 5 1","dig2 3 6"] Explanation: The letter-log contents are all different, so their ordering is "art can", "art zero", "own kit dig". The digit-logs have a relative order of "dig1 8 1 5 1", "dig2 3 6".

Example 2:

Input: logs = ["a1 9 2 3 1","g1 act car","zo4 4 7","ab1 off key dog","a8 act zoo"] Output: ["g1 act car","a8 act zoo","ab1 off key dog","a1 9 2 3 1","zo4 4 7"]

Constraints:

• 1 <= logs.length <= 100
• 3 <= logs[i].length <= 100
• All the tokens of logs[i] are separated by a single space.
• logs[i] is guaranteed to have an identifier and at least one word after the identifier.Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
  1. Open brackets must be closed by the same type of brackets.
  2. Open brackets must be closed in the correct order.
  
  Input: s = "()" Output: trueExample 2:Input: s = "()[]{}" Output: trueExample 3:Input: s = "(]" Output: falseExample 4:Input: s = "([)]" Output: falseExample 5:Input: s = "{[]}" Output: true
  
  • 1 <= s.length <= 104
  • s consists of parentheses only '()[]{}'.
• Constraints:
• Example 1:
• An input string is valid if:

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풀이 1. 스택 일치 여부 판별

def isValid(self, s:str) -> bool:
    stack = []
    table = {
        ')':'(',
        '}':'{',
        ']':'[',
    }

    #스택 이용 예외 처리 및 일치 여부 판별
    for char in s:
        if char not in table:
            stack.append(char)
        elif not stack or table[char] != stack.pop():
            return False
    return len(stack) == 0

문자열 뒤집기

344. Reverse String

https://leetcode.com/problems/reverse-string/

Write a function that reverses a string. The input string is given as an array of characters s.

Example 1:

Input: s = ["h","e","l","l","o"] Output: ["o","l","l","e","h"]

Example 2:

Input: s = ["H","a","n","n","a","h"] Output: ["h","a","n","n","a","H"]

Constraints:

• 1 <= s.length <= 105
• s[i] is a printable ascii character.

Follow up: Do not allocate extra space for another array. You must do this by modifying the input array in-place with O(1) extra memory.

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풀이 1. 투 포인터를 이용한 스왑

def reverseString(self, s: list[str]) -> None:
    left, right =0, len(s) -1
    while left < right:
        s[left], s[right] = s[right], s[left]
        left += 1
        right -= 1

풀이 2. 파이썬 다운 방식

def reverseString(self, s: list[str]) -> None:
    s.reverse()

주식을 사고팔기 가장 좋은 시점

https://leetcode.com/problems/best-time-to-buy-and-sell-stock/

You are given an array prices where prices[i] is the price of a given stock on the ith day.

You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock.

Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.

Example 1:

Input: prices = [7,1,5,3,6,4] Output: 5 Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 = 5. Note that buying on day 2 and selling on day 1 is not allowed because you must buy before you sell.

Example 2:

Input: prices = [7,6,4,3,1] Output: 0 Explanation: In this case, no transactions are done and the max profit = 0.

Constraints:

• 1 <= prices.length <= 105
• 0 <= prices[i] <= 104

풀이 1. 브루트 포스로 계산

def maxProfit(self prices: List[int]) -> int:
    max_price = 0

    for i, price in enumerate(prices):
        for j in range(i, len(prices)):
            max_price = max(pricees[j] - price, max_price)

    return max_price