https://leetcode.com/problems/missing-number/

main.go

package main

import "fmt"

func main() {
	fmt.Println(missingNumber([]int{3,0,1}))
}

func missingNumber(nums []int) int {
	m := make(map[int]struct{}, len(nums))
	for i := 0; i < len(nums)+1; i++ {
		m[i] = struct{}{}
	}

	for _, num := range nums {
		delete(m, num)
	}

	var ret int
	for k, _ := range m {
		ret = k
	}
	return ret
}