First Non Repeating Character Using Hash in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of first non-repeating character index

What is the output of this C code that finds the index of the first non-repeating character in the string?

DSA C

#include <stdio.h>
#include <string.h>

int firstNonRepeatingChar(char *str) {
    int count[256] = {0};
    int i;
    for (i = 0; str[i]; i++) {
        count[(unsigned char)str[i]]++;
    }
    for (i = 0; str[i]; i++) {
        if (count[(unsigned char)str[i]] == 1) {
            return i;
        }
    }
    return -1;
}

int main() {
    char str[] = "swiss";
    int index = firstNonRepeatingChar(str);
    printf("%d\n", index);
    return 0;
}

A 1

B -1

C 0

D 3

💡 Hint

Check the frequency of each character and find the first with count 1.

The string "swiss" has characters: s(3), w(1), i(1). The first non-repeating character is 'w' at index 1.

❓ Predict Output

intermediate

Output of first non-repeating character index with different string

What is the output of this C code that finds the index of the first non-repeating character in the string?

DSA C

#include <stdio.h>
#include <string.h>

int firstNonRepeatingChar(char *str) {
    int count[256] = {0};
    int i;
    for (i = 0; str[i]; i++) {
        count[(unsigned char)str[i]]++;
    }
    for (i = 0; str[i]; i++) {
        if (count[(unsigned char)str[i]] == 1) {
            return i;
        }
    }
    return -1;
}

int main() {
    char str[] = "aabbcc";
    int index = firstNonRepeatingChar(str);
    printf("%d\n", index);
    return 0;
}

A 0

B -1

C 1

D 2

💡 Hint

All characters repeat, so no non-repeating character.

All characters appear twice, so the function returns -1 indicating no non-repeating character.

🔧 Debug

advanced

Identify the error in this code for first non-repeating character

What error will this C code produce when compiled or run?

DSA C

#include <stdio.h>

int firstNonRepeatingChar(char *str) {
    int count[256];
    int i;
    for (i = 0; i < 256; i++) {
        count[i] = 0;
    }
    for (i = 0; str[i]; i++) {
        count[(unsigned char)str[i]]++;
    }
    for (i = 0; str[i]; i++) {
        if (count[(unsigned char)str[i]] == 1) {
            return i;
        }
    }
    return -1;
}

int main() {
    char str[] = "hello";
    int index = firstNonRepeatingChar(str);
    printf("%d\n", index);
    return 0;
}

A Compilation error due to uninitialized array

B Runtime error due to negative array index

C No error, output is 0

D No error, output is 2

💡 Hint

Check how the array is initialized and how characters are used as indices.

The array count is initialized manually with a loop. For "hello", 'h' appears once at index 0, so output is 0. Characters have positive ASCII, no negative index issue. No error.

❓ Predict Output

advanced

Output of first non-repeating character index with mixed case

What is the output of this C code that finds the index of the first non-repeating character in the string with mixed case letters?

DSA C

#include <stdio.h>
#include <string.h>

int firstNonRepeatingChar(char *str) {
    int count[256] = {0};
    int i;
    for (i = 0; str[i]; i++) {
        count[(unsigned char)str[i]]++;
    }
    for (i = 0; str[i]; i++) {
        if (count[(unsigned char)str[i]] == 1) {
            return i;
        }
    }
    return -1;
}

int main() {
    char str[] = "aAbBABac";
    int index = firstNonRepeatingChar(str);
    printf("%d\n", index);
    return 0;
}

A 1

B 6

C -1

D 2

💡 Hint

Remember that uppercase and lowercase letters have different ASCII codes.

String "aAbBABac": a(3), A(2), b(1 at index 2), B(2), c(1). First non-repeating is 'b' at index 2.

🧠 Conceptual

expert

Why use a hash array for first non-repeating character?

Why is using a hash array (like int count[256]) efficient for finding the first non-repeating character in a string?

A Because it counts frequency in O(n) time and uses constant space for ASCII

B Because it sorts the string characters in O(n log n) time

C Because it uses recursion to find unique characters

D Because it stores characters in a linked list for quick access

💡 Hint

Think about how many times the string is scanned and the size of the frequency array.

Using a fixed-size array for ASCII characters allows counting frequencies in one pass (O(n)) and then finding the first unique character in another pass, both linear time. The space is constant (256 ints).