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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0301_0400.s0306_additive_number;

// #Medium #String #Backtracking #2022_07_07_Time_2_ms_(71.98%)_Space_41.9_MB_(69.02%)

/**
 * 306 - Additive Number\.
 *
 * Medium
 *
 * An **additive number** is a string whose digits can form an **additive sequence**.
 *
 * A valid **additive sequence** should contain **at least** three numbers. Except for the first two numbers, each subsequent number in the sequence must be the sum of the preceding two.
 *
 * Given a string containing only digits, return `true` if it is an **additive number** or `false` otherwise.
 *
 * **Note:** Numbers in the additive sequence **cannot** have leading zeros, so sequence `1, 2, 03` or `1, 02, 3` is invalid.
 *
 * **Example 1:**
 *
 * **Input:** "112358"
 *
 * **Output:** true
 *
 * **Explanation:**
 *
 *     The digits can form an additive sequence: 1, 1, 2, 3, 5, 8.
 *     1 + 1 = 2, 1 + 2 = 3, 2 + 3 = 5, 3 + 5 = 8 
 *
 * **Example 2:**
 *
 * **Input:** "199100199"
 *
 * **Output:** true
 *
 * **Explanation:**
 *
 *     The additive sequence is: 1, 99, 100, 199.
 *     1 + 99 = 100, 99 + 100 = 199 
 *
 * **Constraints:**
 *
 * *   `1 <= num.length <= 35`
 * *   `num` consists only of digits.
 *
 * **Follow up:** How would you handle overflow for very large input integers?
**/
public class Solution {
    public boolean isAdditiveNumber(String num) {
        int n = num.length();
        for (int i = 1; i <= n / 2; ++i) {
            for (int j = 1; Math.max(j, i) <= n - i - j; ++j) {
                if (isValid(i, j, num)) {
                    return true;
                }
            }
        }
        return false;
    }

    private boolean isValid(int i, int j, String num) {
        if (num.charAt(0) == '0' && i > 1) {
            return false;
        }
        if (num.charAt(i) == '0' && j > 1) {
            return false;
        }
        String sum;
        Long x1 = Long.parseLong(num.substring(0, i));
        Long x2 = Long.parseLong(num.substring(i, i + j));
        for (int start = i + j; start != num.length(); start += sum.length()) {
            x2 = x2 + x1;
            x1 = x2 - x1;
            sum = x2.toString();
            if (!num.startsWith(sum, start)) {
                return false;
            }
        }
        return true;
    }
}