arrays - 从两个数组/slice 中获取交集和排除项的最有效方法是什么？

Question

给定两个数组或 slice ，例如:

a := []int{1, 2, 3, 4, 5}
b := []int{3, 4, 5, 6, 7, 8, 9}

slice 可能未排序，顺序无关紧要。

计算值的最有效方法是什么，这样您最终得到两个 slice 的公共(public)元素，并且剩余元素存在于一个 slice 中但不存在于另一个 slice 中，即对于上面给出的两个数组，返回值将是:

common := []int{3, 4, 5}
inAButNotB := []int{1, 2}
inBButNotA := []int{6, 7, 8, 9}

计算交集很容易，只需将一个 slice 转换为 map ，然后遍历该 slice 以查看值是否存在。有没有办法在同一个循环中计算其他两个值？

Answer 1

O(len(a) + len(b))是有效的。例如，

package main

import (
    "fmt"
)

func main() {
    a := []int{1, 2, 3, 4, 5}
    b := []int{3, 4, 5, 6, 7, 8, 9}
    fmt.Println(a)
    fmt.Println(b)

    m := make(map[int]uint8)
    for _, k := range a {
        m[k] |= (1 << 0)
    }
    for _, k := range b {
        m[k] |= (1 << 1)
    }

    var inAAndB, inAButNotB, inBButNotA []int
    for k, v := range m {
        a := v&(1<<0) != 0
        b := v&(1<<1) != 0
        switch {
        case a && b:
            inAAndB = append(inAAndB, k)
        case a && !b:
            inAButNotB = append(inAButNotB, k)
        case !a && b:
            inBButNotA = append(inBButNotA, k)
        }
    }
    fmt.Println(inAAndB)
    fmt.Println(inAButNotB)
    fmt.Println(inBButNotA)
}

Playground :https://play.golang.org/p/RvGaC9Wfjiv

输出:

[1 2 3 4 5]
[3 4 5 6 7 8 9]
[3 4 5]
[1 2]
[8 6 7 9]

---

The Go Programming Language Specification

&    bitwise AND            integers
|    bitwise OR             integers
^    bitwise XOR            integers
&^   bit clear (AND NOT)    integers

<<   left shift             integer << unsigned integer
>>   right shift            integer >> unsigned integer

uint8 有 8 位. Bit 0 ( 1 << 0 , 1 shift left 0) is a位 1(1 << 1；1 左移 1)是 b .对于 uint8位，00000001是a , 00000010是b , 00000011是a和 b , 和 00000000是下界a也不b . |运算符设置位，&运算符(operator)读取了一点。

---

The Go Programming Language Specification

Map types

A map is an unordered group of elements of one type, called the element type, indexed by a set of unique keys of another type, called the key type.

The comparison operators == and != must be fully defined for operands of the key type; thus the key type must not be a function, map, or slice. If the key type is an interface type, these comparison operators must be defined for the dynamic key values; failure will cause a run-time panic.

该算法适用于其元素可以是映射键的任何 slice 类型。必须为键类型的操作数完全定义比较运算符 == 和 !=。