【LeetCode】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.equalsIgnoreCase("")||a==null)
            return b;
        if(b.equalsIgnoreCase("")||b==null)
            return a;
        int i=a.length()-1;
        int j=b.length()-1;
        int[] in = new int[Math.max(a.length(), b.length())+1];
        Arrays.fill(in, 0);
        int k=in.length-1;
        while(i>=0&&j>=0){
            in[k]=a.charAt(i)-'0'+b.charAt(j)-'0';
            k--;
            j--;
            i--;
        }
        if(i==-1){
            while(j>=0){
                in[k]=b.charAt(j)-'0';
                j--;
                k--;
            }
        }
        if(j==-1){
            while(i>=0){
                in[k]=a.charAt(i)-'0';
                i--;
                k--;
            }
        }
        int pre = 0;
        for(int m=in.length-1;m>=0;m--){
            int temp = in[m]+pre;
            pre = temp/2;
            in[m]=temp%2;
        }
        StringBuilder sb = new StringBuilder();
        int init=0;
        if(in[0]==0)
            init=1;
        while(init<in.length){
            sb.append(in[init]);
            init++;
        }
        
        return sb.toString();
        
    }
}