代码随想录二刷(哈希表)

三数之和思路反正对于我来说是真的难想出来。

若这道题还是采用哈希表的思路去做，非常麻烦，并且还要考虑去重的操作。所以这道题其实用双指针，是更方便的。

具体程序如下：

class Solution:
    def threeSum(self, nums: List[int]) -> List[List[int]]:
        result = []
        nums.sort()
        for i in range(len(nums)):
            if nums[i] > 0:return result
            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:right -= 1
                elif sum < 0:left += 1
                else:
                    result.append([nums[i],nums[left],nums[right]])
                    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

下面来看下四数之和：

四数之和相当于三数之和升级了一下，并且target不是0了。按照三数之和的思路，再加一个变量作为外层循环。组合出来四个数即可。

class Solution:
    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
        result = []
        n = len(nums)
        nums.sort()
        for i in range(n):
            if nums[i] > target and target > 0:break 
            if i > 0 and nums[i] == nums[i-1]:
                continue
            for j in range(i+1,n):
                if (nums[i] + nums[j]) > target and target > 0:break 
                if j > i + 1 and nums[j] == nums[j-1]:
                    continue
                left = j + 1
                right = n - 1
                while right > left:
                    sum = nums[i] + nums[j] + nums[left] + nums[right]
                    if sum > target:right -= 1
                    elif sum < target:left += 1
                    else:
                        result.append([nums[i],nums[j],nums[left],nums[right]])
                        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