Add Binary

Given two binary strings, return their sum (also a binary string).

For example,
a = "11"
b = "1"
Return "100".

思路： 从后向前加，保持进位，当无进位，且遍历完毕时结束。

注意：Collections.reverse不能用于String，而StringBuffer自己带有reverse函数，return a reference to this object.

1. public String addBinary(String a, String b) {
3. if(a==null) return b;
4. if(b==null) return a;
5. int alen = a.length();
6. int blen = b.length();
7. if(alen==0) return b;
8. if(blen==0) return a;
9. StringBuffer res = new StringBuffer();
10. int cnt = 0;
11. alen = alen-1;
12. blen = blen -1;
13. while(alen>=0 || blen>=0 || cnt>0) {
14. int first = 0;
15. int second=0;
16. if(alen>=0) first=a.charAt(alen--)-'0';
17. if(blen>=0) second=b.charAt(blen--)-'0';
18. int sum = first + second + cnt;
19. if(sum>=2) {
20. sum = sum - 2;
21. cnt = 1;
22. } else {
23. cnt = 0;
24. }
25. res.append(sum);
26. }
27. String res2 = res.reverse().toString();
28. return res2;
29. }