update XYDP

A Recursion an DP problem from the Cracking. Very Easy! Can somebody
show me the DP solution using iterative not recursion. 拜谢！
Imagine a robot sitting on the upper left corner of an X by Y grid. The
robot can only move in two directions: right and down. How many possible
paths are there for the robot go from (0, 0) to (X, Y)?