Email Validator - Problem

You are given a string and need to validate if it represents a valid email format.

A valid email must satisfy all of the following conditions:

Contains exactly one '@' symbol
Has a domain part after the '@' (at least one character)
Contains no spaces anywhere in the string
Uses only valid characters: letters (a-z, A-Z), digits (0-9), dots (.), hyphens (-), underscores (_)
Does not start or end with a dot

Return true if the email is valid, false otherwise.

Input & Output

Example 1 — Valid Email

$ Input: email = "user@domain.com"

› Output: true

💡 Note: Has exactly one '@' at position 4, domain 'domain.com' exists, no spaces, all characters are valid (letters, dot), doesn't start or end with dot

Example 2 — Invalid: Multiple @ Symbols

$ Input: email = "user@@domain.com"

› Output: false

💡 Note: Contains two '@' symbols, which violates the rule of exactly one '@' symbol

Example 3 — Invalid: Contains Spaces

$ Input: email = "user @domain.com"

› Output: false

💡 Note: Contains a space character before '@', which is not allowed in valid email format

Constraints

1 ≤ email.length ≤ 1000
email consists of printable ASCII characters

Visualization

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Asked in

M Microsoft 35 G Google 28 f Facebook 22 a Amazon 18

The key insight is to validate all email rules in a single pass through the string. The optimal approach scans once while tracking @ symbol count, character validity, and position constraints simultaneously. Time: O(n), Space: O(1)

Common Approaches

✓ Multiple Validation Checks

⏱️ Time: O(n) Space: O(1)

Perform separate checks for each validation rule: count @ symbols, find domain, check for spaces, validate characters, and check start/end conditions. Each check requires scanning parts or all of the string multiple times.

Single Pass Validation

⏱️ Time: O(n) Space: O(1)

Scan through the string once and check all validation rules simultaneously. Keep track of @ count, character validity, spaces, and positions in one efficient pass.

Multiple Validation Checks — Algorithm Steps

Count @ symbols in entire string
Split by @ and check domain exists
Scan for spaces in entire string
Check each character is valid
Verify first and last characters aren't dots

Visualization

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Step-by-Step Walkthrough

Count @ Symbols

Scan entire string to count @ occurrences

Check Domain

Split by @ and verify both parts exist

Validate Characters

Check each character against valid set

Final Checks

Verify no spaces, proper start/end

Code -

solution.c — C

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

bool isValidChar(char c) {
    return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || 
           (c >= '0' && c <= '9') || c == '.' || c == '-' || c == '_';
}

bool solution(char* email) {
    if (email == NULL || strlen(email) == 0) {
        return false;
    }
    
    int len = strlen(email);
    int atCount = 0;
    int atPos = -1;
    
    // Check for exactly one @ symbol and find its position
    for (int i = 0; i < len; i++) {
        if (email[i] == '@') {
            atCount++;
            atPos = i;
        }
    }
    if (atCount != 1 || atPos == 0 || atPos == len - 1) {
        return false;
    }
    
    // Check for spaces and valid characters
    for (int i = 0; i < len; i++) {
        if (email[i] == ' ') {
            return false;
        }
        if (!isValidChar(email[i]) && email[i] != '@') {
            return false;
        }
    }
    
    // Check start and end
    if (email[0] == '.' || email[len-1] == '.') {
        return false;
    }
    
    return true;
}

int main() {
    char email[1000];
    fgets(email, sizeof(email), stdin);
    email[strcspn(email, "\n")] = 0;
    
    bool result = solution(email);
    printf(result ? "true\n" : "false\n");
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Multiple passes through the string, but each pass is O(n), so overall O(n)

✓ Linear Growth

Space Complexity

O(1)

Only using a few variables for counting and flags

✓ Linear Space

23.4K Views

Medium Frequency

~15 min Avg. Time

845 Likes

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Smart Actions

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Email Validator - Problem

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Multiple Validation Checks — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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