ZOJ Problem Set - 2253
Given an m*n grid, you're required to fill it with the numbers from 1 to m*n but the following rules should be satisfied:
1. each number in grid is larger than its above;
Now, I wonder how many distinct solutions I could make on a grid.
For example. Consider such a 2*3 grid, we have the following solutions:
123 124 134 135 125 456 356 256 246 346
Thus, the sum is 5.
Two positive integer m, n (0 < m, n < 10) per line. Two zeros indicate the end of input.
A single integer per line for each case -- the number of distinct solutions.
Author: HE, Tao
Source: ZOJ Monthly, November 2004