#### 1. 替换空格
```python
class Solution:
def replaceSpace(self, s: str) -> str:
res = []
for c in s:
if c == ' ': res.append("%20")
else: res.append(c)
return "".join(res)
```
#### 2. 从尾到头打印链表
```python
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, x):
# self.val = x
# self.next = None
# 方法1：递归
class Solution:
def reversePrint(self, head: ListNode) -> List[int]:
return self.reversePrint(head.next) + [head.val] if head else []
# 方法2：栈
class Solution:
def reversePrint(self, head: ListNode) -> List[int]:
stack = []
while head:
stack.append(head.val)
head = head.next
return stack[::-1]
```
#### 3. 用两个栈实现队列
```python
class CQueue:
def __init__(self):
self.A, self.B = [], []
def appendTail(self, value: int) -> None:
self.A.append(value)
def deleteHead(self) -> int:
if self.B: return self.B.pop()
if not self.A: return -1
while self.A:
self.B.append(self.A.pop())
return self.B.pop()
```
#### 4. 判断字符串是否表示数值
**解题思路：**
本题使用有限状态自动机。根据字符类型和合法数值的特点，先定义状态，再画出状态转移图，最后编写代码即可
