Question
You're given a list of numbers. You need to check whether any value appears more than once.
Return true if any number is repeated. Otherwise, return false.
Example
Input: [1, 2, 3, 4]
Output: false
Explanation: All numbers are unique.
Input: [3, 1, 2, 3]
Output: true
Explanation: The number 3 appears twice.
Input: [7]
Output: false
Explanation: Only one element.
My Notes
- We can use a hash map to track what we've seen.
- Loop through the list and add items to the map.
- If a number already exists in the map, return
true. - This uses O(n) time and O(n) space.
- Map value can be
struct{}since we don’t care about the value — just presence.
Why use struct{} and not bool?
- Zero allocation:
struct{}uses 0 bytes — more memory-efficient thanbool. - Clarity: Makes it explicit we're only tracking presence.
- Idiomatic Go: Common pattern for building "sets" using maps.
- Safer: No risk of logic mistakes from stored
true/falsevalues.
package main
import (
"fmt"
)
func main() {
result := containsDuplicate([]int{1,2,3,4,4,5,6,7})
fmt.Println(result)
}
func containsDuplicate(nums []int) bool {
lookup := make(map[int]struct{})
for _, item := range nums {
if _, ok := lookup[item]; ok {
return true
}
lookup[item] = struct{}{}
}
return false
}