LeetCode259.三数之和较小值 $ 3Sum Smaller

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?

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给定n个整数nums和一个目标的数组，找出满足条件nums[i]+nums[j]+nums[k]<目标的索引三元组i、j、k的个数，其中0<=i<j<k<n。

例如，给定nums=[-2，0，1，3]和target=2。

返回2。因为有两个三元组的和小于2：

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

进阶：

你能在O（n2）运行时解决它吗？

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 1 class Solution {
 2     func threeSumSmaller(_ nums:[Int],_ target:Int) -> Int{
 3         var nums = nums
 4         if nums.count < 3 {return 0}
 5         var res:Int = 0
 6         var n:Int = nums.count
 7         nums = nums.sorted(by:<)
 8         for i in 0..<(n - 2)
 9         {
10             var left:Int = i + 1
11             var right:Int = n - 1
12             while (left < right)
13             {
14                 if nums[i] + nums[left] + nums[right] < target
15                 {
16                     res += right - left
17                     left += 1
18                 }
19                 else
20                 {
21                     right -= 1
22                 }
23             }
24         }
25         return res
26     }
27 }