import java.util.Vector;

class Solution {
    public String addBinary(String a, String b) {
    	//準備過程
    	String res = "";
        a = reverse(a);//將字串反序好加
        b = reverse(b);//將字串反序好加
        int alen = a.length();
        int blen = b.length();
        int carry = 0;
        int temp = 0;
        Vector<Integer> vector = new Vector<Integer>();
        
        //開始加，用一個迴圈完成，手動模擬二進位制加法
        for(int i = 0; i < alen || i < blen; i++){
        	temp = carry;//一開始將carry加在這裡
        	if(i < alen){
        		temp += a.charAt(i) - '0';// + carry;//carry只能加一次
        	}
        	if(i < blen){
        		temp += b.charAt(i) - '0';// + carry;//carry只能加一次
        	}
        	carry = temp / 2;
        	temp = temp % 2;
        	vector.add(temp);
        }
        if(carry != 0) vector.add(carry);
        
        //結果處理過程
        char[] trans = new char[vector.size()];
        int count = vector.size() - 1;
        for(int x : vector){
        	trans[count--] = (char) (x + '0');
        }
        res = res.valueOf(trans);
        return res;
    }
    //編寫的字串反序函式
    public String reverse(String s){
    	char[] res = new char[s.length()];
    	for(int i = s.length() - 1; i >= 0; i--){
    		res[s.length() - 1 - i] = s.charAt(i);
    	}
    	String ss = "";
    	return ss.valueOf(res);
    }
}
 

程式碼優化（演算法思路同“上面程式碼中的開始加，用一個迴圈完成，手動模擬二進位制加法”）

class Solution {
    public String addBinary(String a, String b) {
        StringBuilder sb = new StringBuilder();
        int i = a.length() - 1, j = b.length() -1, carry = 0;
        while (i >= 0 || j >= 0) {
            int sum = carry;
            if (j >= 0) sum += b.charAt(j--) - '0';
            if (i >= 0) sum += a.charAt(i--) - '0';
            sb.append(sum % 2);
            carry = sum / 2;
        }
        if (carry != 0) sb.append(carry);
        return sb.reverse().toString();
    }
}