(This article was first published on

**YGC » R**, and kindly contributed to R-bloggers)Starting in the top left corner of a 2x2 grid, there are 6 routes (without backtracking) to the bottom right corner. How many routes are there through a 20x20 grid?

Using recursive algorithm can solved this problem well. For optimized the running time, I use a matrix to cache previously called functions, as I did in Problem 164.

^{?}View Code RSPLUS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 |
find.routes.internal <- function(x,y) { cnt <- cacheMat[x+1,y+1] if (cnt != 0) return(cnt) if (y >0) cnt <- cnt+find.routes.internal(x,y-1) if (x >0) cnt <- cnt+find.routes.internal(x-1,y) if (x ==0 && y ==0) cnt <- cnt+1 cacheMat[x+1,y+1] <<- cnt return(cnt) } find.routes <- function(x,y) { cacheMat <- matrix(0, nrow=x+1, ncol=y+1) cnt <- find.routes.internal(x,y) return(cnt) } |

#### Related Posts

To

**leave a comment**for the author, please follow the link and comment on his blog:**YGC » R**.R-bloggers.com offers

**daily e-mail updates**about R news and tutorials on topics such as: visualization (ggplot2, Boxplots, maps, animation), programming (RStudio, Sweave, LaTeX, SQL, Eclipse, git, hadoop, Web Scraping) statistics (regression, PCA, time series, trading) and more...