67. Add Binary

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

For example,

a = “11”

b = “1”

Return “100”.

public class Solution {
public String addBinary(String a, String b) {
if(a == null || a.length() == 0) return b;
if(b == null || b.length() == 0) return a;
StringBuilder sb = new StringBuilder();
int la = a.length(),lb = b.length(),add = 0;
int i = la-1,j = lb-1;
while(i >= 0 || j >= 0 || add != 0){
int ta = i >= 0?a.charAt(i)-'0':0;
int tb = j >= 0?b.charAt(j)-'0':0;
int tr = ta^tb^add;
add = (ta+tb+add) > 1?1:0;      //进位
sb.append(tr);
i--;
j--;
}
return sb.reverse().toString();
}
}