1、两数之和

解法一 暴力法（2个for循环）

class Solution:
    def twoSum(self, nums: List[int], target: int) -> List[int]:
        for ii in range(len(nums)):
            for jj in range(ii+1, len(nums)):
                if nums[ii]+nums[jj] == target:
                    return [ii,jj]

解法二 哈希表法

class Solution:
    def twoSum(self, nums: List[int], target: int) -> List[int]:
        hash = {}
        for ii, num in enumerate(nums):
            if target-num in hash:
                return [hash[target-num],ii]
            hash[num] = ii

哈希表[key]=value
key是数值
value是索引值
便历索引，数值用枚举enumerate；
2、两数相加

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        # 初始化链表ListNode()表示链表，head表示静止表头，tree表示动态指针；val是相加后/10的余数，tmp是商；
        head = tree = ListNode()
        val = tmp = 0
        # 当三者有一个不为空时，继续循环
        while tmp or l1 or l2:
            val = tmp
            if l1:
                val = l1.val + val
                l1 = l1.next
            if l2:
                val = l2.val + val
                l2 = l2.next
            
            tmp = val // 10
            val = val % 10

            tree.next = ListNode(val)
            tree = tree.next
        return head.next