First Non-Repeating Value

Find the first input value whose final frequency is one.

Algorithm

Canonical input [3, 5, 2, 5, 3, 8, 2] prints 8. The replay uses the same input in every language, so this C DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.c

#include <stdio.h>

int main(void) {
    int arr[] = {3, 5, 2, 5, 3, 8, 2};
    int count[10] = {0};
    int n = 7;
    for (int i = 0; i < n; i++) {
        count[arr[i]] += 1;
    }
    for (int i = 0; i < n; i++) {
        if (count[arr[i]] == 1) {
            printf("%d\n", arr[i]);
            break;
        }
    }
}

Complexity

Time: O(n) for this checked C source
Space: O(1) for the fixed count[10] direct-address table

Implementation notes

This checked C source is not a real hash table. It uses direct addressing with int count[10] = {0}, so the input values must be valid indexes into that fixed count array.
int arr[] = {3, 5, 2, 5, 3, 8, 2} and count[10] are local stack arrays; n is the literal 7, and there is no helper function where array parameters decay to pointers.
There is no hash function, bucket array, sentinel, or collision handling here: count[arr[i]] += 1 maps each value directly to its count slot.
The first pass mutates counts in input order, producing trace states from {3: 1} through {3: 2, 5: 2, 2: 2, 8: 1}.
The second pass scans the original array order and checks count[arr[i]] == 1; the trace skips 3, 5, 2, 5, and 3, then finds 8 at i=5.
printf("%d\n", arr[i]) prints 8 and breaks. Visible memory is the two stack arrays and scalar loop indexes; visible mutation is confined to count.

two-pass lookup The first pass builds a frequency table. The second pass keeps the original order and stops at the first value with frequency one.