# project euler – Problem 15

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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

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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) } |

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