C++

題目

The count-and-say sequence is a sequence of digit strings defined by the recursive formula:

    - countAndSay(1) = "1"
    - countAndSay(n) is the way you would "say" the digit string from countAndSay(n-1), which is then converted into a different digit string.

To determine how you "say" a digit string, split it into the minimal number of groups so that each group is a contiguous section all of the same character. Then for each group, say the number of characters, then say the character. To convert the saying into a digit string, replace the counts with a number and concatenate every saying.

Given a positive integer n, return the nth term of the count-and-say sequence.

Example:

Input: n = 1
Output: "1"
Explanation: This is the base case.

Input: n = 4
Output: "1211"
Explanation:
countAndSay(1) = "1"
countAndSay(2) = say "1" = one 1 = "11"
countAndSay(3) = say "11" = two 1's = "21"
countAndSay(4) = say "21" = one 2 + one 1 = "12" + "11" = "1211"

題目

Given a sorted array nums, remove the duplicates in-place such that each element appears only once and returns the new length.

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

Clarification:
Confused why the returned value is an integer but your answer is an array?

Note that the input array is passed in by reference, which means a modification to the input array will be known to the caller as well.

Example:

Input: nums = [1,1,2]
Output: 2, nums = [1,2]
Explanation: Your function should return length = 2, with the first two elements of nums being 1 and 2 respectively. It doesn't matter what you leave beyond the returned length.

題目輸入一個由小到大排序的陣列，我們需要回傳『沒有重複元素陣列長度』，並將這些沒有重複的元素移動至該陣列的開頭。

當然你可以選擇刪除掉重複的元素，但驗證時只會依照你回傳的長度進行該陣列的檢測。比方說你回傳 1，那麼驗證機制只會檢測陣列開頭第 1 個元素。

題目

Given the head of a linked list, remove the nth node from the end of the list and return its head.

Follow up: Could you do this in one pass?

Example:

Input: head = [1,2,3,4,5], n = 2
Output: [1,2,3,5]

本題給定一個鏈結串列（Linked List），而我們要做的，便是將倒數第 n 個元素刪除，並回傳刪除後的 Linked List。

題目

Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

Notice that the solution set must not contain duplicate quadruplets.

Example:

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

這個題目是 2Sum、3Sum 的衍伸版，同樣給定一個整數陣列輸入，我們要找出由四個數加總為 0 的所有組合。

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。