mirror of
https://github.com/TheAlgorithms/C
synced 2025-04-20 20:32:50 +03:00
3c8f86e740
* feat: add Letter combinations of phone book problem (#17) * fix: add newline at the end of the file * fix: add brief description of the algorithm --------- Co-authored-by: David Leal <halfpacho@gmail.com>
79 lines
2.4 KiB
C
79 lines
2.4 KiB
C
/**
|
|
* Letter Combinations of a Phone Number problem
|
|
* The algorithm determines the final size of the return array (combs) and allocates
|
|
* corresponding letter for each element, assuming that the return array is alphabetically sorted.
|
|
* It does so by running two loops for each letter:
|
|
* - the first loop determines the starting positions of the sequence of subsequent letter positions
|
|
* - the second loop determines the length of each subsequent sequence for each letter
|
|
* The size and space complexity are both O("size of final array"), as even though there are 4 loops,
|
|
* each element in the final array is accessed only once.
|
|
*/
|
|
|
|
#include <stdlib.h> // for the malloc() function
|
|
#include <string.h> // for the strlen() function
|
|
|
|
char *get_letters(char digit) {
|
|
switch (digit) {
|
|
case '2':
|
|
return "abc";
|
|
case '3':
|
|
return "def";
|
|
case '4':
|
|
return "ghi";
|
|
case '5':
|
|
return "jkl";
|
|
case '6':
|
|
return "mno";
|
|
case '7':
|
|
return "pqrs";
|
|
case '8':
|
|
return "tuv";
|
|
case '9':
|
|
return "wxyz";
|
|
default:
|
|
return "";
|
|
}
|
|
}
|
|
|
|
char **letterCombinations(char *digits, int *return_size) {
|
|
char *cp;
|
|
int i, j, k, l, ind, k_tot, l_tot, digits_size = 0;
|
|
|
|
if (*digits == '\0') {
|
|
*return_size = 0;
|
|
return NULL;
|
|
}
|
|
|
|
*return_size = 1;
|
|
cp = digits;
|
|
while (*cp != '\0') {
|
|
*return_size *= strlen(get_letters(*cp));
|
|
digits_size++;
|
|
cp++;
|
|
}
|
|
|
|
char **combs = malloc(sizeof(char*) * (*return_size));
|
|
for (i = 0; i < *return_size; i++) {
|
|
combs[i] = malloc(sizeof(char) * (digits_size + 1));
|
|
combs[i][digits_size] = '\0';
|
|
}
|
|
|
|
k_tot = 1;
|
|
l_tot = (*return_size);
|
|
for (i = 0; i < digits_size; i++) { // loop accross digits
|
|
cp = get_letters(digits[i]);
|
|
l_tot /= strlen(cp);
|
|
for (j = 0; j < strlen(cp); j++) { // loop accross letters of the digit
|
|
for (k = 0; k < k_tot; k++) { // loop across the subset starting positions for each letter
|
|
for (l = 0; l < l_tot; l++) { // loop accross each subset positions for each letter
|
|
ind = k * l_tot * strlen(cp) + l + l_tot * j;
|
|
combs[ind][i] = cp[j];
|
|
}
|
|
}
|
|
}
|
|
k_tot *= strlen(cp);
|
|
}
|
|
|
|
return combs;
|
|
}
|