The CS Detective

—17—A Binary Search Tree Trap

Less than a block from Cloaks and More, Frank spotted a woman tailing him. Even as irritation flashed through him, he had to admit that she was good. She kept to the other side of the road, always at least 30 feet back. More often than not, she relied on the reflections in the shop windows to watch him. And she wore a perfectly nondescript traveling cloak—a shade of forest green sported by over half the people on the street.

Frank stopped abruptly, dropped to one knee, and pretended to tie his shoe. It was the second oldest technique for identifying a tail, the first being to dash madly in a random direction and see who followed. Although arguably less effective than a mad dash, the fake-shoe-tying routine had the ...