15. 三数之和

单纯三层遍历会超时卡在最后三个测试用例，参考题解解法。

执行用时：116 ms, 在所有 Swift 提交中击败了99.44%的用户
内存消耗：17.1 MB, 在所有 Swift 提交中击败了100.00%的用户

题目链接：https://leetcode-cn.com/problems/3sum/

class Solution {
    func threeSum(_ nums: [Int]) -> [[Int]] {
        var sortnums = nums.sorted()
        let n = sortnums.count
        var first = 0, second = 0, third = 0
        var res = [[Int]]()
        for first in 0 ..< n {
            if first > 0 && sortnums[first] == sortnums[first-1] {
                continue
            }
            third = n - 1
            second = first + 1
            for second in second ..< n {
                if second > first + 1 && sortnums[second] == sortnums[second-1] {
                    continue
                }
                while second < third && sortnums[first] + sortnums[second] + sortnums[third] > 0 {
                    third -= 1
                }
                // 指针重合说明下一轮循环找不到更小的c使得a+b+c=0
                if second == third {
                    break
                }
                if sortnums[first] + sortnums[second] + sortnums[third] == 0 {
                    res.append([sortnums[first], sortnums[second], sortnums[third]])
                }
            }
        }
        return res
    }
}