Diagonal array walker

diagwalk.c

#include <stdlib.h>

int* getDiagonalOrder(int** matrix, int m, int n) {
  int *r = malloc(m * n * sizeof(int));
  int dir = 1, i = 0, j = 0;
  for (int k = 0; k < m * n; ++k) {
    r[k] = matrix[i][j];
    if (dir > 0) {
      if (j == n - 1) {
        dir = -1;
        ++i;
      } else if (i == 0) {
        dir = -1;
        ++j;
      } else {
        --i;
        ++j;
      }
    } else {
      if (i == m - 1) {
        dir = 1;
        ++j;
      } else if (j == 0) {
        dir = 1;
        ++i;
      } else {
        --j;
        ++i;
      }
    }
  }
  return r;
}