Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    int sum = 0, sumsq = 0;
    int sumreal =0;
    int sumsqreal = 0;
    for(int i=0; i<n; i++){
        sum += i;
        sumsq += (i*i);
        sumreal += arr[i];
        sumsqreal += (arr[i]*arr[i]);
    }
    int a = (sum-sumreal)>=0? (sum - sumreal):(sumreal-sum);
    int b = (sumsq-sumsqreal)>=0? (sumsq - sumsqreal):(sumsqreal-sumsq);
    int num1 = (a + b/a)/2;
    int num2 = (-a + b/a)/2;
    int freq1 = 0, freq2 = 0;
    for(int i=0; i<n; i++){
        if(arr[i]==num1) freq1++;
        if(arr[i]==num2) freq2++;
    }
    return freq1==2? num1:num2;
}

int main() {
    int n;
    scanf("%d", &n);

    int arr[100];
    for (int i = 0; i < n; i++) scanf("%d", &arr[i]);

    int result = find_duplicate(arr, n);
    printf("%d", result);
    return 0;
}

Solving Approach