<p>Given two binary strings <code>a</code> and <code>b</code>, return <em>their sum as a binary string</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> a = "11", b = "1"
<strong>Output:</strong> "100"
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> a = "1010", b = "1011"
<strong>Output:</strong> "10101"
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= a.length, b.length &lt;= 10<sup>4</sup></code></li>
	<li><code>a</code> and <code>b</code> consist&nbsp;only of <code>&#39;0&#39;</code> or <code>&#39;1&#39;</code> characters.</li>
	<li>Each string does not contain leading zeros except for the zero itself.</li>
</ul>

class Solution {
public:
    string addBinary(string a, string b) {
        int alen=a.length();
        int blen=b.length();
        int i=0, carry=0;
        string ans="";

        while(i<alen || i<blen || carry !=0){
            int x=0;
            if(i<alen && a[alen-i-1]=='1')
            {
                x=1;
            }
            int y=0;
            if(i<blen and b[blen-i-1]=='1')
            {
                y=1;
            }
            ans=to_string((x+y+carry)%2) + ans;
            carry=(x+y+carry)/2;
            i+=1;
        }
        return ans;



        }

        
    };

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