$y=int(x)$ is not one-to-one.
Work Step by Step
*Condition for One-to-one Function (the Horizontal Line Test): The graph of function $y=f(x)$ intersects each horizontal line at most once. Looking at the graph of $y=int(x)$, we notice that the graph is actually a collection of short horizontal lines. Therefore, if we draw any horizontal lines through any short horizontal line of the given graph, it would intersect with infinite number of points there. Therefore, we conclude that $y=int(x)$ is not one-to-one.