每日面试算法题: Ways to Traverse a Grid

Hi, here's your problem today. This problem was recently asked by Microsoft:

You 2 integers n and m representing an n by m grid, determine the number of ways you can get from the top-left to the bottom-right of the matrix y going only right or down.

Example:
n = 2, m = 2

This should return 2, since the only possible routes are:
Right, down
Down, right.

Here's the signature:

def num_ways(n, m):

  # Fill this in.



print num_ways(2, 2)

# 2