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。