NO.15 三数之和

#双指针，注意排除重复值，比如[2,2]那么指针就要往前或者后移
def threeSum(self, nums: [int]) -> [[int]]:
    nums.sort()#从小到大排序
    res, k = [], 0
    for k in range(len(nums) - 2):
        if nums[k] > 0: break # 1. 因为排序后设置k为最小值若其大于0，则无解
        if k > 0 and nums[k] == nums[k - 1]: continue # 2. 跳过相同的值nums[k]
        i, j = k + 1, len(nums) - 1
        while i < j: # 3. 双指针临界条件
            s = nums[k] + nums[i] + nums[j] 
            if s < 0:
                i += 1
                while i < j and nums[i] == nums[i - 1]: i += 1 # 4.1 s<0右移动
            elif s > 0:
                j -= 1
                while i < j and nums[j] == nums[j + 1]: j -= 1 # 4.2 s>0左移动
            else:
                res.append([nums[k], nums[i], nums[j]])
                i += 1
                j -= 1
                while i < j and nums[i] == nums[i - 1]: i += 1 #左指针遇到重复值
                while i < j and nums[j] == nums[j + 1]: j -= 1 #右指针遇到重复值
    return res