原题链接15. 3Sum
Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note:

The solution set must not contain duplicate triplets.

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
[-1, 0, 1],
[-1, -1, 2]
]
解题思路：转化为n个2sum问题

func threeSum(nums []int) [][]int {
    var res [][]int
    sort.Ints(nums)
    lg := len(nums)
    for i := 0; i < lg-2; i++ {
        l := i + 1
        r := lg - 1
        if i > 0 && nums[i] == nums[i-1] {
            continue
        }
        for l < r {
            if nums[i]+nums[l]+nums[r] == 0 {
                res = append(res, []int{nums[i], nums[l], nums[r]})
                l++
                r--
                for l < r && nums[l] == nums[l-1] {
                    l++
                }
                for l < r && nums[r] == nums[r+1] {
                    r--
                }
            } else if nums[i]+nums[l]+nums[r] < 0 {
                l++
            } else {
                r--
            }
        }
    }
    return res
}