3340. Check Balanced String

Method 1

You are given a string num consisting of only digits. A string of digits is called balanced if the sum of the digits at even indices is equal to the sum of digits at odd indices.
Return true if num is balanced, otherwise return false.

Example 1

Input: num = "1234"

Output: false

Explanation:

The sum of digits at even indices is 1 + 3 == 4, and the sum of digits at odd indices is 2 + 4 == 6.
Since 4 is not equal to 6, num is not balanced.

Example 2

Input: num = "24123"

Output: true

Explanation:

The sum of digits at even indices is 2 + 1 + 3 == 6, and the sum of digits at odd indices is 4 + 2 == 6.
Since both are equal the num is balanced.

Method 1

【O(n) time | O(1) space】

package Leetcode.Math;

/**
 * @author zhengxingxing
 * @date 2025/03/16
 */
public class CheckBalancedString {
    
    public static boolean isBalanced(String num) {
        // Variable to store sum of digits at even indices (0,2,4...)
        int evenSum = 0;
        // Variable to store sum of digits at odd indices (1,3,5...)
        int oddSum = 0;

        // Iterate through each character in the string
        for (int i = 0; i < num.length(); i++) {
            if (i % 2 == 0) {
                // For even indices, add digit to evenSum
                // Subtract '0' to convert char to corresponding integer
                evenSum += num.charAt(i) - '0';
            } else {
                // For odd indices, add digit to oddSum
                oddSum += num.charAt(i) - '0';
            }
        }

        // Return true if sums are equal, false otherwise
        return evenSum == oddSum;
    }

    /**
     * Main method containing test cases
     * Tests the isBalanced method with various inputs
     */
    public static void main(String[] args) {
        // Test Case 1: Regular case where string is not balanced
        String num1 = "1234";
        System.out.println("Test Case 1: " + num1);
        System.out.println("Expected Result: false");
        System.out.println("Actual Result: " + isBalanced(num1));

        // Test Case 2: Regular case where string is balanced
        String num2 = "24123";
        System.out.println("\nTest Case 2: " + num2);
        System.out.println("Expected Result: true");
        System.out.println("Actual Result: " + isBalanced(num2));

        // Test Case 3: Edge case - empty string
        String num3 = "";
        System.out.println("\nTest Case 3: " + num3);
        System.out.println("Expected Result: true");
        System.out.println("Actual Result: " + isBalanced(num3));

        // Test Case 4: Edge case - single digit
        String num4 = "1";
        System.out.println("\nTest Case 4: " + num4);
        System.out.println("Expected Result: true");
        System.out.println("Actual Result: " + isBalanced(num4));
    }
}

Method 1

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