Reverse the order of words in a string

Q: Write a C program to reverse the order of words in a string. For simplicity, space has been chosen as word delimiter and punctuation is considered part of the word.


Example: 
Input: Can you reverse the words in this string.
Output: string. this in words the reverse you Can


Time Complexity: O(log(n))


void reverse_word(char *word, int length)
{
    int start;
    int end = length - 1;
    char temp;


    for (start = 0; start < end; start++, end--) {
        temp = word[start];
        word[start] = word[end];
        word[end] = temp;
    }
}


void reverse_string(char *string)
{
    int index;
    int string_length = strlen(string);
    int word_start_index = 0;
    int word_length = 0;


    // Reverse the original string
    reverse_word(string, string_length);


    for (index = 0; index < string_length; index++) {
        // found word delimiter
        if (string[index] == ' ') {
            reverse_word(&string[word_start_index], word_length);
            word_length = 0;
            word_start_index = index + 1;
        } else {
            word_length++;
        }
    }


    // Reverse the last word
    if (word_length) {
        reverse_word(&string[word_start_index], word_length);
    }
}

Palindrome

Question: Write a program to find whether a given string is a palindrome or not. [A palindrome is a word, phrase, number or other alpha-numeric sequence that can be read the same way in either direction]
Time complexity: O(n)


bool
palindrome (char *str)
{
    int len = (strlen(str) - 1);
    int count = 0;


    for (count = 0; count < len; count++, len--) {
        if (str[count] != str[len]) {
            return (FALSE);
        }
    }


    return (TRUE);
}

String Permutation

Question: Write a program to display all permutation of alphabets in the string ? For example: string 'xyz' has following possible permutations:
xyz
xzy
yxz
yzx
zxy
zyx

// initial call: permutations("xyz", 0)

void permutations(char *str, int level)
{
    int i = 0;
    int len = strlen(str);<br>

    if (level == len)

    {
        // Termination Condition
        printf("%s\n", output);
        return;
    }


    for (i = 0; i < len; i++)
    {
        if (flag[i]) continue;
        output[level] = str[i];
        flag[i] = 1;
        permutations(str, level + 1);
        flag[i] = 0;
    }
}

String Combination

Question: Write a program to display all combinations of alphabets  in a given string ? For example: string 'xyz' has following possible combinations:
x
xy
xyz
xz
y
yz
z

// initial call: combinations("xyz", 0, 0)

void combinations(char *str, int start, int level)

{

    int i = 0;
    int len = strlen(str);

    if (start == len) {

        // Termination Condition

        return;

    }

    for(i = start; i < len; i++)

    {

        output[level] = str[i];

        output[level + 1] = '\0';

        printf("%s\n", output);

        combinations(str, i + 1, level + 1);

    }

}