代码随想录算法训练营第6天|454，383，15（重写），18（

四数相加II
学到了
class Solution:
def fourSumCount(self, nums1: List[int], nums2: List[int], nums3: List[int], nums4: List[int]) -> int:
hashmap = dict()
for i in nums1:
for j in nums2:
if i+j in hashmap:
hashmap[i+j] += 1
else:
hashmap[i+j] = 1

count = 0
for n3 in nums3:
for n4 in nums4:
key = - n3 - n4
if key in hashmap:
count += hashmap[key]
return count

赎金信
class Solution:
def canConstruct(self, ransomNote: str, magazine: str) -> bool:
hashmap = defaultdict(int)

for x in magazine:
hashmap[x] += 1

for x in ransomNote:
value = hashmap.get(x)
if not value:
return False
else:
hashmap[x] -= 1

return True

三数之和
自己写的话，写了个循环，还是卡在去重上了，需要重点再练一下
class Solution:
def threeSum(self, nums: List[int]) -> List[List[int]]:
result = []
nums.sort()

for i in range(len(nums)):
    # 如果第一个元素已经大于0，不需要进一步检查
    if nums[i] > 0:
        return result
    
    # 跳过相同的元素以避免重复
    if i > 0 and nums[i] == nums[i - 1]:
        continue
        
    left = i + 1
    right = len(nums) - 1
    
    while right > left:
        sum_ = nums[i] + nums[left] + nums[right]
        
        if sum_ < 0:
            left += 1
        elif sum_ > 0:
            right -= 1
        else:
            result.append([nums[i], nums[left], nums[right]])
            
            # 跳过相同的元素以避免重复
            while right > left and nums[right] == nums[right - 1]:
                right -= 1
            while right > left and nums[left] == nums[left + 1]:
                left += 1
                
            right -= 1
            left += 1
            
return result

四数之和
需要重写
class Solution:
def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
nums.sort()
n = len(nums)
result = []
for i in range(n):
if nums[i] > target and nums[i] > 0 and target > 0:# 剪枝（可省）
break
if i > 0 and nums[i] == nums[i-1]:# 去重
continue
for j in range(i+1, n):
if nums[i] + nums[j] > target and target > 0: #剪枝（可省）
break
if j > i+1 and nums[j] == nums[j-1]: # 去重
continue
left, right = j+1, n-1
while left < right:
s = nums[i] + nums[j] + nums[left] + nums[right]
if s == target:
result.append([nums[i], nums[j], nums[left], nums[right]])
while left < right and nums[left] == nums[left+1]:
left += 1
while left < right and nums[right] == nums[right-1]:
right -= 1
left += 1
right -= 1
elif s < target:
left += 1
else:
right -= 1
return result