Given a string email that denotes an Email Address, the task is to check if the given string is a valid email id or not. If found to be true, then print “Valid”. Otherwise, print “Invalid”. A valid email address consists of an email prefix and an email domain, both in acceptable formats:

• The email address must start with a letter (no numbers or symbols).
• There must be an @ somewhere in the string that is located before the dot.
• There must be text after the @ symbol but before the dot.
• There must be a dot and text after the dot.

Examples:

Input: email = “gyaanibuddy@gmail.com” 
Output: Valid
Explanation:
The given string follow all the criteria for an valid email string.

Input: email = “gyaanibuddy@gmail...com”
Output: Invalid

Input: email = “gyaanibuddy@@gmail.com”
Output: Invalid

1. Check if the first character of the email id string is an alphabet or not. If not, then the email is Invalid.
2. Now traverse over the string email to find the position the “@” and “.” If “@” or “.” is not present then the email is Invalid.
3. If “.” is not present after “@” then the email is Invalid.
4. If “.” is the last character of the string email then the email id is Invalid.
5. Otherwise, the email is Valid.

Code

// C++ program for the above approach

#include <bits/stdc++.h>
using namespace std;

// Function to check the character
// is an alphabet or not
bool isChar(char c)
{
	return ((c >= 'a' && c <= 'z')
			|| (c >= 'A' && c <= 'Z'));
}

// Function to check the character
// is an digit or not
bool isDigit(const char c)
{
	return (c >= '0' && c <= '9');
}

// Function to check email id is
// valid or not
bool is_valid(string email)
{
	// Check the first character
	// is an alphabet or not
	if (!isChar(email[0])) {

		// If it's not an alphabet
		// email id is not valid
		return 0;
	}
	// Variable to store position
	// of At and Dot
	int At = -1, Dot = -1;
    int countAt = 0,countDot = 0;
	// Traverse over the email id
	// string to find position of
	// Dot and At
	for (int i = 0;
		i < email.length(); i++) {

		// If the character is '@'
		if (email[i] == '@') {
            countAt++;
			At = i;
		}

		// If character is '.'
		else if (email[i] == '.') {
          countDot++;
			Dot = i;
		}
	}

	// If At or Dot is not present
	if (At == -1 || Dot == -1)
		return 0;

	// If Dot is present before At
	if (At > Dot)
		return 0;
	//If Dot or At is present more than one  .
     if(countAt > 1 || countDot > 1 ){
         return 0;
     }
	// If Dot is present at the end
	return !(Dot >= (email.length() - 1));
}

// Driver Code
int main()
{
	// Given string email
	string email = "gyaanibuddy@gmail.com";

	// Function Call
	bool ans = is_valid(email);

	// Print the result
	if (ans) {
		cout << email << " : "
			<< "valid" << endl;
	}
	else {
		cout << email << " : "
			<< "invalid" << endl;
	}

	return 0;
}

Output:

gyaanibuddy@gmail.com : valid

Time Complexity: O(N)
Auxiliary Space: O(1)