# 9am

## What’s The Number Again ?

In Algorithms, Graphs, Programming, Strings on September 16, 2010 at 9:49 pm
The previous post “What’s The Number” had a non-recursive implementation. Here is a recursive way of doing the same. It’s not as bad to have a recursive implementation as the max recursion depth is limited by the length of  a telephone number. If the input was longer / without an upper bound, recursion would be a bad move. This Recursive algorithm does a Depth First walk. A recursive Breadth First walk is much more painful to implement and has a higher memory overhead.
/*
Given a telephone number like 4156434531, get all possible names that would match it.
e.g. If the number is 23, the names could be “ad”, “bd”, “cd”, “ae” etc..
*/
public static class TelNumber
{
private static char[][] KeyChars = new char[10][];
static TelNumber()
{
KeyChars[0] = new char[]{‘+’};
KeyChars[1] = new char[]{‘ ‘};
KeyChars[2] = new char[]{‘a’,’b’,’c’};
KeyChars[3] = new char[]{‘d’,’e’,’f’};
KeyChars[4] = new char[]{‘g’,’h’,’i’};
..
}
public static void PrintCombination(StringBuffer prefix, int[] tel_no)
{
int index = prefix.Count;
if (index >= tel_no.Length)
{
System.Console.Writeline(prefix);
return;
}
foreach(char c in KeyChars[tel_no[index]])
{