LeetCode - 2. Add Two Numbers

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        String s1 = toStr(l1);
        String s2 = toStr(l2);
        return toNode(addTowStr(s1, s2));
    }
    
    private String toStr(ListNode node) {
        StringBuilder sr = new StringBuilder();
        if (node == null)
            return sr.toString();
        while (node != null) {
            sr.append(node.val);
            node = node.next;
        }
        return sr.toString();
    }
    
    private ListNode toNode(String str) {
        if (str == null)
            return null;
        
        ListNode cur = new ListNode(str.charAt(0)-‘0‘);
        ListNode head = cur;
        for (int i=1; i<str.length(); i++) {
            cur.next = new ListNode(str.charAt(i)-‘0‘);
            cur = cur.next;
        }
        return head;
    }
    
    private String addTowStr(String s1, String s2) {
        if (s1 == null || s1.length() <= 0 || s1.equals("0"))
            return s2;
        if (s2 == null || s2.length() <= 0 || s2.equals("0"))
            return s1;
        
        if (s1.length() < s2.length()) {
            String t = s1;
            s1 = s2;
            s2 = t;
        }
        int minLen = s2.length();
        int maxLen = s1.length();
        int over = 0;
        StringBuilder ret = new StringBuilder();
        for (int i=0,j=0; i<=minLen-1 && j<=minLen; i++,j++) {
            int sum = (s1.charAt(j)-‘0‘) + (s2.charAt(i)-‘0‘) + over;
            if (sum < 10) {
                ret.append(sum+"");
                over = 0;
            }
            else {
                ret.append((sum % 10)+"");
                over = sum / 10;
            }
        }
        
        for (int i=minLen; i<maxLen; i++) {
            int sum = (s1.charAt(i)-‘0‘) + over;
            if (sum < 10) {
                ret.append(sum+"");
                over = 0;
            }
            else {
                over = sum / 10;
                ret.append((sum % 10)+"");
            }
        }
        if (over > 0)
            ret.append(over);
        return ret.toString();
    }
    
    
}