Leetcode 259. 3Sum Smaller

 class Solution(object):
     def threeSumSmaller(self, nums, target):
         """
         :type nums: List[int]
         :type target: int
         :rtype: int
         """
         nums.sort()
         n = len(nums)
         ans = 0

         for k in range(n):
             i, j = k+1, n-1
             while i<j:
                 sum3 = nums[k] + nums[i] + nums[j]
                 if sum3 < target:
                     ans += j - i
                     i += 1
                 else:
                     j -= 1
         return ans

Given an array of n integers nums and a target, find the number of index triplets i, j, k with 0 <= i < j < k < n that satisfy the condition nums[i] + nums[j] + nums[k] < target.

For example, given nums = [-2, 0, 1, 3], and target = 2.

Return 2. Because there are two triplets which sums are less than 2:

[-2, 0, 1]
[-2, 0, 3]

Follow up:
Could you solve it in O(n²) runtime?

思路： 相遇型two pointers。注意L17, 这一句的意思是，如果 nums[k] + nums[i] + nums[j] < target，那么i和j之间的所有nums[p]+nums[k]+nums[i]也都满足条件。