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LeetCode: 32-Longest Valid Parentheses 解題紀錄

題目

Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

Example:

Input: s = "(()"
Output: 2
Explanation: The longest valid parentheses substring is "()".

Input: s = ")()())"
Output: 4
Explanation: The longest valid parentheses substring is "()()".

這個題目輸入一個只由 '(' 或是 ')' 組成的字串，而我們要做的，就是判斷最長的『合理』子字串長度。

比方說， "()" 就是合理的子字串、")(" 就是個不合理的子字串。

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LeetCode: 16-3Sum Closest 解題紀錄

題目

Given an array nums of n integers and an integer target, find three integers in nums such that the sum is closest to target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

Example:

Input: nums = [-1,2,1,-4], target = 1
Output: 2
Explanation: The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

Constraints:

  • 3 <= nums.length <= 10^3
  • -10^3 <= nums[i] <= 10^3
  • -10^4 <= target <= 10^4

這個題目跟 15 題非常相似，同樣是給定一組整數陣列輸入，不過還多給了一個目標值 target。我們需要找出 3 個數值，使其加總值越接近 target 越好。

需要注意的是，我們要返回的並不是找到的陣列，而是直接返回最接近 target 的數值。

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LeetCode: 15-3Sum 解題紀錄

題目

Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Notice that the solution set must not contain duplicate triplets.

Example:

Input: nums = [-1,0,1,2,-1,-4]
Output: [[-1,-1,2],[-1,0,1]]

Input: nums = []
Output: []

題目是給定一個全是由『整數』組成的陣列，而我們的目標就是要找出『任意三個數值加總為 0 的所有答案』，也是題目 3Sum 的意義。

還有，返回的結果不能有重複的答案。

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LeetCode: 31-Next Permutation 解題紀錄

題目

Implement next permutation, which rearranges numbers into the lexicographically next greater permutation of numbers.

If such an arrangement is not possible, it must rearrange it as the lowest possible order (i.e., sorted in ascending order).

The replacement must be in place and use only constant extra memory.

Example:

Input: nums = [1,2,3]
Output: [1,3,2]

Input: nums = [3,2,1]
Output: [1,2,3]

Input: nums = [1,1,5]
Output: [1,5,1]

Input: nums = [1]
Output: [1]

題目給定輸入一個陣列，我們要將陣列排列成『下一個較大的值』。比方說題目輸入了 [1,2,3]，那麼我們就不能排列成 [3,1,2]、而是要排列成 [1,3,2] —— 因為 132 才下一個較大的值，而非 312。

而當題目給定的陣列沒有下一個較大的排列時，則將陣列『從小排到大』。

順帶一提這題不會返回任何值，直接處理輸入陣列即可。

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LeetCode: 13-Roman to Integer 解題紀錄

題目

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.
Symbol       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000
For example, 2 is written as II in Roman numeral, just two one's added together. 12 is written as XII, which is simply X + II. The number 27 is written as XXVII, which is XX + V + II.

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used:
  • I can be placed before V (5) and X (10) to make 4 and 9. 
  • X can be placed before L (50) and C (100) to make 40 and 90. 
  • C can be placed before D (500) and M (1000) to make 400 and 900.
Given a roman numeral, convert it to an integer.

Example:

Input: s = "LVIII"
Output: 58
Explanation: L = 50, V= 5, III = 3.

Input: s = "MCMXCIV"
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.
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LeetCode: 11-Container With Most Water 解題紀錄

題目

Given n non-negative integers a1, a2, ..., an , where each represents a point at coordinate (i, ai). n vertical lines are drawn such that the two endpoints of the line i is at (i, ai) and (i, 0). Find two lines, which, together with the x-axis forms a container, such that the container contains the most water.

Notice that you may not slant the container.

Example:

Input: height = [1,8,6,2,5,4,8,3,7]
Output: 49
Explanation: The above vertical lines are represented by array [1,8,6,2,5,4,8,3,7]. In this case, the max area of water (blue section) the container can contain is 49.

這個題目也是相當地經典，就是題目給定一陣列，陣列內為由左到右每個 ...... ㄜ，這裡就叫『木板』好了，每個木板的高度。

而不同的木板之間，距離單位都是 1。我們要做的就是，找出能讓容器積蓄最多的水兩片木板，最後，回傳可以積蓄水的最大值。

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LeetCode: 12-Integer to Roman 解題紀錄

題目

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

Symbol       Value
I             1
V             5
X             10
L             50
C             100
D             500
M             1000

For example, 2 is written as II in Roman numeral, just two one's added together. 12 is written as XII, which is simply X + II. The number 27 is written as XXVII, which is XX + V + II.

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used:

-  I can be placed before V (5) and X (10) to make 4 and 9. 
-  X can be placed before L (50) and C (100) to make 40 and 90. 
-  C can be placed before D (500) and M (1000) to make 400 and    900.

Given an integer, convert it to a roman numeral.

Example:

Input: num = 3
Output: "III"

Input: num = 58
Output: "LVIII"

Input: num = 1994
Output: "MCMXCIV"

題目給定一整數值，而我們要做的便是將整數值轉換成羅馬數字 —— 也就是說，我們需要回傳字串型態的答案。

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LeetCode: 14-Longest Common Prefix 解題紀錄

題目

Write a function to find the longest common prefix string amongst an array of strings.

If there is no common prefix, return an empty string "".

Example:

Input: strs = ["flower","flow","flight"]
Output: "fl"

Input: strs = ["dog","racecar","car"]
Output: ""

題目給定一陣列，裡頭存放許多組字串，而我們要做的就是找到他們共同的前綴字串並回傳；若無共同的前綴，則回傳空字串 ""。

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