What is the genre of the story "After Twenty Years" by O. Henry?

O. Henry's "After Twenty Years" falls under the genre of Realism.
Set in New York City at the turn of the twentieth century, this tale of the fated reunion of two old friends after twenty years, and involves some surprising contradictions. It begins with a policeman who walks his beat by checking the doorways and the store doors. As he does so, he comes to the dark doorway of what was once a restaurant and finds a man standing there.

"It's all right, officer," he said, reassuringly. "I'm just waiting for a friend. It's an appointment made twenty years ago."

While the stranger explains, he lights a cigar. As he does so, the light reveals a scar near the man's right eyebrow and a large diamond in his necktie. These distinguishing marks identify the man as the policeman's old friend, who is now known as 'Silky Bob' and wanted by police in Chicago. When Jimmy Wells, the policeman, recognizes his old friend as a wanted man, he does not have the heart to arrest the ill-fated man. After ascertaining that Bob will allow his friend thirty minutes before he leaves the doorway, Wells returns to the police station and asks a plainclothes policeman if he will make the arrest for him as he does not have the heart to do so.
In the end, while there is the ironic and fateful meeting of policeman Jimmy Wells and the wanted man 'Silky Bob,' his old friend, such a happenstance is not completely unrealistic.