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.length() < b.length()){
String tmp = a;
a = b;
b = tmp;
}
int pa = a.length()-1;
int pb = b.length()-1;
int carries = 0;
String rst = "";
while(pb >= 0){
int sum = (int)(a.charAt(pa) - '0') + (int)(b.charAt(pb) - '0') + carries;
rst = String.valueOf(sum % 2) + rst;
carries = sum / 2;
pa --;
pb --;
}
while(pa >= 0){
int sum = (int)(a.charAt(pa) - '0') + carries;
rst = String.valueOf(sum % 2) + rst;
carries = sum / 2;
pa --;
}
if (carries == 1)
rst = "1" + rst;
return rst;
}
}