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

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

public String addBinary(String a, String b) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int carry = 0;
        String sumStr = "";
        int i=a.length()-1;
        int j=b.length()-1;
        while(i>=0 || j>=0){
            int bitSum =0;
            if(i>=0 && j>=0){
                bitSum= Integer.parseInt(a.substring(i,i+1))+Integer.parseInt(b.substring(j,j+1)) + carry;
                i--;
                j--;
            }else if(i>=0){
                bitSum= Integer.parseInt(a.substring(i,i+1))+carry;
                i--;
            }else{
                bitSum= Integer.parseInt(b.substring(j,j+1)) + carry;
                j--;
            }
            carry = bitSum/2;
            int bit = bitSum%2;
            sumStr = bit+sumStr;    
        }
        if(carry==1){
            sumStr = carry+sumStr;
        }
        return sumStr;
    }

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