03 Non-Repeating Element

Non-Repeating Element | Practice | GeeksforGeeks

Question

Find the first non-repeating element in a given array arr of N integers. Note: Array consists of only positive and negative integers and not zero.

Example 1:

Input : arr[] = {-1, 2, -1, 3, 2}
Output : 3
Explanation:
-1 and 2 are repeating whereas 3 is 
the only number occurring once.
Hence, the output is 3. 

Example 2:

Input : arr[] = {1, 1, 1}
Output : 0

Expected Time Complexity: $O(N)$ Expected Auxiliary Space: $O(N)$

Constraints:

$1 <= N <= 10^7$

$-10^{16} <= A_i <= 10^{16}$​

${A_i \ne0 }$​

Using map

Logic

1. Create a map of frequencies

2. Traverse the array from left to right

   1. If the element has frequency 1 in the map then return the element

3. If no such element is found, return -1

Code

int firstNonRepeating(int arr[], int n) 
{ 
    map<int, int> frequency;
    
    for(int i = 0; i < n; i++)
        frequency[arr[i]]++;
        
    for(int i = 0; i < n; i++)
        if(frequency[arr[i]] == 1)
            return arr[i];
            
    return -1;
} 

Complexity

Time Complexity: $O(n)$​

Space Complexity: $O(n)$​