Leetcode - Additive Number

My code:

import java.math.BigInteger;

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(i, j) <= 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 = "";
        BigInteger x1 = new BigInteger(num.substring(0, i));
        BigInteger x2 = new BigInteger(num.substring(i, i + j));
        for (int start = i + j; start != num.length(); start += sum.length()) {
            x2 = x2.add(x1);
            x1 = x2.subtract(x1);
            sum = x2.toString();
            if (!num.startsWith(sum, start)) {
                return false;
            }
        }
        return true;
    }
}

reference:
https://discuss.leetcode.com/topic/29856/java-recursive-and-iterative-solutions

解释的很好。

Anyway, Good luck, Richardo! -- 10/14/2016