算法学习笔记2：双指针

2. 双指针

• 相同数组，不同数组
• 两边指针，快慢指针

题目：977. 有序数组的平方

代码模板：

class Solution:
    def sortedSquares(self, nums: List[int]) -> List[int]:
        # 双指针，绝对值最大的在2边
        i, j = 0, len(nums)-1
        res = [0] * len(nums)
        k = j
        while k > -1:
            if nums[i]**2 < nums[j]**2:
                res[k] = nums[j]**2
                j -= 1
            else:
                res[k] = nums[i]**2
                i += 1
            k -= 1
        return res

题目：189. 轮转数组（重点题）

代码模板：

class Solution:
    def rotate(self, nums: List[int], k: int) -> None:
        """
        Do not return anything, modify nums in-place instead.
        """
        # 法2，使用额外空间
        n = len(nums)
        idx = n - k%n
        res = [0] * n
        i = j = 0
        while i < n:
            res[i] = nums[(i+idx) % n]
            i += 1
        while j < n:
            nums[j] = res[j]
            j += 1

题目：283. 移动零（思路很清晰）

代码模板：

class Solution:
    def moveZeroes(self, nums: List[int]) -> None:
        """
        Do not return anything, modify nums in-place instead.
        """
        # 同向指针，不同条件判断是否右移
        i = j = 0
        while j < len(nums):
            if nums[j] != 0:
                # 左侧换完之后保证都是非0数
                nums[i], nums[j] = nums[j], nums[i]
                i += 1
            j += 1

题目：167. 两数之和 II - 输入有序数组

代码模板：

class Solution:
    def twoSum(self, numbers: List[int], target: int) -> List[int]:
        # 首尾双指针，常量空间
        i, j = 0, len(numbers)-1
        while i < j:
            if numbers[i] + numbers[j] == target:
                return [i+1, j+1]
            elif numbers[i] + numbers[j] > target:
                j -= 1
            else:
                i += 1
        return [-1, -1]

题目：557. 反转字符串中的单词 III

代码模板：

class Solution:
    def reverseWords(self, s: str) -> str:
        # 将s转变为列表，原地修改
        s = list(s)
        i = j = 0
        while j < len(s):
            while j < len(s) and s[j] != ' ':
                j += 1
            # 反转s[i:j+1]部分
            k = j
            j -= 1
            while i < j:
                s[i], s[j] = s[j], s[i]
                i += 1
                j -= 1
            j = k+1
            i = k+1
        return ''.join(s)

伪原地算法

题目：876. 链表的中间结点

代码模板：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def middleNode(self, head: ListNode) -> ListNode:
        # 快慢指针
        low, fast = head, head
        while fast and fast.next:
            low = low.next
            fast = fast.next.next
        return low

题目：19. 删除链表的倒数第 N 个结点

代码模板：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def removeNthFromEnd(self, head: ListNode, n: int) -> ListNode:
        # 伪头，n在要求范围内
        ori = ListNode
        ori.next = head
        slow = fast = head
        while n > 0:
            fast = fast.next
            n -= 1
        slow = ori
        while fast:
            slow = slow.next
            fast = fast.next
        slow.next = slow.next.next
        return ori.next

因为删除当前节点需要其上一个节点，所以添加哑节点