src/main/java/com/fishercoder/solutions/_306.java

package com.fishercoder.solutions;

public class _306 {
    public static class Solution1 {
        /**
         * Credit: https://discuss.leetcode.com/topic/29856/java-recursive-and-iterative-solutions/2
         */
        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;
        }
    }

}