Nikhil Learn Hub - String Problem Solving Tricks in C

Essential String Manipulation Tricks

#	Trick	Description
1	Two-pointer on strings	Use two indices (start and end) to compare or manipulate string efficiently
2	Character frequency counting	Use an int freq[256] = {0} to count each character using ASCII values
3	In-place string reversal	Swap characters using two pointers, no extra string needed
4	Removing duplicates	Mark visited chars with a boolean array or use slow-fast pointers
5	String tokenization	Use strtok() to split string based on delimiters like spaces, commas
6	String rotation check	Concatenate string with itself and check if other string is a substring
7	Substring search (Brute/Optimized)	Use nested loops for brute, or implement KMP/Rabin-Karp
8	String to number / number to string	Use atoi(), itoa() or manual conversion via loop
9	Prefix/Suffix matching	Compare beginning or end characters using strncmp() or manual loop
10	Reversing words in string	First reverse entire string, then reverse each word
11	Palindrome subsequence / substring	Check longest palindromic subsequence or substring
12	Sliding window on strings	Maintain frequency window to find smallest/largest substrings
13	Lexicographic operations	Use strcmp(), sorting, etc., to compare alphabetically
14	Edit distance / DP on strings	Use 2D DP table to compute min operations to convert strings
15	Bit masking for lowercase letters	Use 26-bit integer to track lowercase letters efficiently

Implementation Examples

Two-pointer Example

// Palindrome check using two pointers
int isPalindrome(char *s) {
    int left = 0, right = strlen(s) - 1;
    while (left < right) {
        if (s[left++] != s[right--]) 
            return 0;
    }
    return 1;
}

Frequency Counting

// Count character frequencies
void countChars(char *s) {
    int freq[256] = {0};
    for (int i = 0; s[i]; i++) {
        freq[(int)s[i]]++;
    }
    // Print frequencies
    for (int i = 0; i < 256; i++) {
        if (freq[i]) printf("%c: %d\n", i, freq[i]);
    }
}

In-place Reversal

// Reverse string in-place
void reverseString(char *s) {
    int n = strlen(s);
    for (int i = 0, j = n-1; i < j; i++, j--) {
        char temp = s[i];
        s[i] = s[j];
        s[j] = temp;
    }
}

Sliding Window

// Longest substring without repeating chars
int lengthOfLongestSubstring(char *s) {
    int freq[256] = {0}, left = 0, max_len = 0;
    for (int right = 0; s[right]; right++) {
        freq[s[right]]++;
        while (freq[s[right]] > 1) {
            freq[s[left++]]--;
        }
        max_len = fmax(max_len, right-left+1);
    }
    return max_len;
}