Permutations and Combinations

一些算法：http://www.cs.utexas.edu/users/djimenez/utsa/cs3343/lecture25.html

1. Print all permutations of a string.

一位网友的博文：http://n1b-algo.blogspot.com/2009/01/string-permutations.html

2. Given a telephone number, find all the permutations of the letters.

char letters[10][5] = 
{
	" ",	//0
	" ",	//1
	"ABC",	//2
	"DEF",	//3
	"GHI",	//4
	"JKL",	//5
	"MNO",	//6
	"PQRS",	//7
	"TUV",	//8
	"WXYZ"	//9
};
int num_letters[10] = {1,1,3,3,3,3,3,4,3,4};
int answer[10];
void print_result(string num)
{
	for(int i=0; i<10; i++)
	{
		cout<<letters[num[i]-'0'][answer[i]];
	}
	cout<<endl;
}
void num_letter(string num, int level)
{
	if(level == 10)
	{
		print_result(num);
		return;
	}
	for(int i=0; i<num_letters[num[level]-'0']; i++)
	{
		answer[level]=i;
		num_letter(num, level+1);
	}
}