How to get the intersection of two slices in Go

Background

• It is a common requirement to get the common elements between two slices, but I found that some people still use double loops to implement it during Code Review. (The time complexity is O(n^2), and the efficiency is relatively low)
• I have been using Golang recently, so I will share how to get the common elements between two slices in Golang.

❌ Code example implemented with double loops:

func intersection(nums1 []int, nums2 []int) []int {
    var result []int

    // Double loop O(n^2)
    for _, v1 := range nums1 { // O(n)  outer loop
        for _, v2 := range nums2 { // O(n) inner loop
            if v1 == v2 {
                result = append(result, v1)
            }
        }
    }
    return result
}

Improvement

Change the element lookup in the inner loop in the above example to be implemented using set*,

so that the time complexity of the inner loop part can be reduced to O(1), and the overall time complexity can be reduced to O(n).

Set in Javascript

Set objects are collections of values. A value in the set may only occur once; it is unique in the set's collection.

It could be represented internally as a hash table (with O(1) lookup),

a search tree (with O(log(N)) lookup), or any other data structure, as long as the complexity is better than O(N).

Reference

Implementation example using set

func intersection(nums1 []int, nums2 []int) []int {
    var result []int

    // Convert nums2 to set, so that the time complexity of looking up elements in nums2 becomes O(1).
    //  Consider performance optimization, you can compare the number of elements in nums1 and nums2, and convert the slice with more elements into set.
    set := make(map[int]struct{}) // There is no set in Golang, use map to implement. struct{} is an empty structure to save memory.
    for _, v := range nums2 {
        set[v] = struct{}{}
    }

    // Traverse nums1, if the element in nums1 exists in nums2, add it to result
    for _, v := range nums1 {
        if _, ok := set[v]; ok {
            result = append(result, v)
        }
    }
}

Implementation using third-party libraries

If you consider performance optimization and various types of slices, we can use the following third-party libraries to implement.

• deckarep/golang-set
• samber/lo

deckarep/golang-set

As the name suggests, it is a set implementation in Golang.

import (
  "fmt"
  mapset "github.com/deckarep/golang-set/v2"
)

func main() {
    set1 := mapset.NewSet[string]()
    set1.Add("a")
    set1.Add("b")
    set1.Add("c")

    set2 := mapset.NewSet[string]()
    set2.Add("c")
    set2.Add("d")
    set2.Add("e")

    // 交集 intersection
    intersectionSet := set1.Intersect(set2)
    fmt.Println(intersectionSet) // Set{c}

    // Besides intersection, it also supports union, difference, symmetric difference, etc.
    // 并集 union
    unionSet := set1.Union(set2)
    fmt.Println(unionSet) // Set{a, b, c, d, e}

    // 差集 difference
    diffSet := set1.Difference(set2)
    fmt.Println(diffSet) // Set{a, b}

    // 对称差集 symmetric difference
    symDiffSet := set1.SymmetricDifference(set2)
    fmt.Println(symDiffSet) // Set{a, b, d, e}
    }

samber/lo

If you need to sort, group, etc. in addition to intersecting slices, you can consider using the samber/lo library.

It is similar to lodash in Javascript.

import (
    "github.com/samber/lo"
)

func main() {
    // 交集 intersection
    lo.Intersection([]int{1, 2, 3}, []int{2, 3, 4}) // return []int{2, 3}

    // 并集 union
    lo.Union([]int{1, 2, 3}, []int{2, 3, 4}) //return []int{1, 2, 3, 4}

    // 差集 difference
    lo.Difference([]int{1, 2, 3}, []int{2, 3, 4}) // return []int{1}, []int{4}
}