#include <stddef.h>

#define COUNTS_LEN 128

int find_odd (size_t length, const int array[length]) {
  // counts of positive numbers
  int pos_counts[COUNTS_LEN];
  // counts of negative numbers; indices are absolute values
  int neg_counts[COUNTS_LEN];

  // initialize counts arrays to -1
  for (int i = 0; i < COUNTS_LEN; i++)
    neg_counts[i] = pos_counts[i] = -1;

  // count up numbers
  for (int i = 0; i < length; i++) {
    int n = array[i];

    if (n < 0) {
      if (neg_counts[-n] < 0)
        neg_counts[-n] = 0;
      neg_counts[-n] += 1;
    } else {
      if (pos_counts[n] < 0)
        pos_counts[n] = 0;
      pos_counts[n] += 1;
    }
  }

  // check pos counts
  for (int i = 0; i < COUNTS_LEN; i++)
    if (pos_counts[i] >= 0 && pos_counts[i] % 2 != 0)
      return i;

  // check neg counts
  for (int i = 0; i < COUNTS_LEN; i++)
    if (neg_counts[i] >= 0 && neg_counts[i] % 2 != 0)
      return -i;

  return 0;
}