1346. Check If N and Its Double Exist

Easy
Given an array arr of integers, check if there exist two indices i and j such that :
  • i != j
  • 0 <= i, j < arr.length
  • arr[i] == 2 * arr[j]
Example 1:
Input: arr = [10,2,5,3]
Output:
true
Explanation:
For i = 0 and j = 2, arr[i] == 10 == 2 * 5 == 2 * arr[j]
Example 2:
Input: arr = [3,1,7,11]
Output:
false
Explanation:
There is no i and j that satisfy the conditions.
Constraints:
  • 2 <= arr.length <= 500
  • -103 <= arr[i] <= 103

解題

O(n)
使用 map 紀錄出現過的數,並檢查當前數字的兩倍或除以二(偶數情況)的數是否已經出現過。
func checkIfExist(arr []int) bool {
m := make(map[int]bool)
​
for _, num := range arr {
if (num % 2 == 0 && m[num/2]) || (m[num * 2]) {
return true
}
​
m[num] = true
}
​
return false
}