Topic 20: Valid Brackets
Given a string that only includes'(',')','{','}','[',']', determine whether the string is valid.
A valid string must satisfy: the
left parenthesis must be closed with the same type of right parenthesis.
The opening parenthesis must be closed in the correct order.
Note that an empty string can be considered a valid string.
answer:
class Solution:
def isValid(self, s: str) -> bool:
while '()' in s or '[]' in s or '{}' in s:
s = s.replace('()','')
s = s.replace('[]','')
s = s.replace('{}','')
return s == ''
Operation result:
Topic 21: Combine two ordered linked
lists. Combine two ascending linked lists into a new ascending linked list and return. The new linked list is composed by splicing all the nodes of the given two linked lists.
answer:
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next
class Solution:
def mergeTwoLists(self, l1: ListNode, l2: ListNode) -> ListNode:
res = ListNode(None)
node = res
while l1 and l2:
if l1.val < l2.val:
node.next,l1 = l1,l1.next
else:
node.next,l2 = l2,l2.next
node = node.next
if l1:
node.next = l1
else:
node.next = l2
return res.next
Operation result:
Topic 16: The sum of the nearest three numbers.
Given an array nums including n integers and a target value target. Find the three integers in nums so that their sum is closest to target. Returns the sum of these three numbers. Assume that there is only one answer for each set of inputs.
answer:
class Solution:
def threeSumClosest(self, nums: List[int], target: int) -> int:
nums, r, end = sorted(nums), float('inf'), len(nums) - 1
for c in range(len(nums) - 2):
i, j = max(c + 1, bisect.bisect_left(nums, target - nums[end] - nums[c], c + 1, end) - 1), end
while r != target and i < j:
s = nums[c] + nums[i] + nums[j]
r, i, j = min(r, s, key=lambda x: abs(x - target)), i + (s < target), j - (s > target)
return r
operation result:
