【重点】【双指针】18.四数之和

题目
跟三数之和思路完全一致，都是双指针问题

双指针

Python

最朴素版本代码！

class Solution:
    def fourSum(self, nums: List[int], target: int) -> List[List[int]]:
        res = []
        if len(nums) < 4:
            return res
        n = len(nums)
        nums.sort()
        for a in range(n-3):
            if a > 0 and nums[a] == nums[a-1]: continue
            for b in range(a+1, n-2):
                if b > a + 1 and nums[b] == nums[b-1]: continue
                c, d = b+1, n-1
                while c < d:
                    if nums[c] + nums[d] < target - nums[a] - nums[b]:
                        c += 1
                    elif nums[c] + nums[d] > target - nums[a] - nums[b]:
                        d -= 1
                    else:
                        res.append([nums[a], nums[b], nums[c], nums[d]])
                        c += 1
                        d -= 1
                        while c < d and nums[c] == nums[c-1]:
                            c += 1
                        while c < d and nums[d] == nums[d+1]:
                            d -= 1
        
        return res