67. Add Binary

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

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

// String.valueOf(1) 可以转换int 1为String类型的”1″
// 将较长的数字放到前面，可以简化代码
// 强制转换成了int也简化了4种if的情况

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;
    }
}