306. Additive Number

public class Solution {
    public boolean isAdditiveNumber(String num) {
        int l = num.length();
        if(l<3)
            return false;
        
        if(num.startsWith("00")) //Then the whole string should be "000..."
        {
            for(int i=2;i<l;++i)
            {
                if(num.charAt(i) != ‘0‘)
                    return false;
            }
            return true;
        }
        
        int firstNumEndIndex = (l-1)/2;
        if(num.startsWith("0")) //Then 0 must be the first interger.
            firstNumEndIndex = 1;
        for(int i = 1; i<= firstNumEndIndex; ++i)
        {
            String add1 = num.substring(0, i); //[0, i) => first number
            for(int j = i+1; j<= l - Math.max(i,j-i); ++j)
            {
                String add2 = num.substring(i, j);//[i, j) => second number
                if(add2.equals("0"))
                {
                    if(rollToEnd(num, add1, add2, j))
                        return true;
                    break;
                }    
                if(rollToEnd(num, add1, add2, j))
                    return true;
            }
        }
        return false;
    }
    
    
    //[start,end)
    private boolean rollToEnd (String num, String add1, String add2, int from)
    {
        String sumString = getSumString(add1, add2);
        String rest = num.substring(from);
        if(rest.startsWith("0"))
            return false;
        if(rest.equals(sumString))
            return true;
        if(rest.startsWith(sumString))
            return rollToEnd(num, add2, sumString, from+sumString.length());
        return false;
    }
    
    private String getSumString(String add1, String add2)
    {
        int extra = 0;
        int l1 = add1.length();
        int l2 = add2.length();
        Deque<Integer> sum = new ArrayDeque<Integer>();
        
        while(l1>0 && l2>0)
        {
            int a1 = add1.charAt(l1-1) - ‘0‘;
            int a2 = add2.charAt(l2-1) - ‘0‘;
            int s = a1+a2+extra;
            extra = s/10;
            sum.push(s%10);
            --l1;
            --l2;
        }
        
        while(l1>0)
        {
            int a = add1.charAt(l1-1) - ‘0‘;
            int s = a + extra;
            extra = s/10;
            sum.push(s%10);
            --l1;
        }
        
        while(l2>0)
        {
            int a = add2.charAt(l2-1) - ‘0‘;
            int s = a + extra;
            extra = s/10;
            sum.push(s%10);
            --l2;
        }
        
        if(extra == 1)
            sum.push(extra);
        
        StringBuilder sb = new StringBuilder();
        while(!sum.isEmpty())
            sb.append(sum.pop());
        return sb.toString();
    }
}