<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 na=a.length();
        int nb=b.length();
        int m=max(na,nb);
        a=string(m-na+1,'0')+a;
        b=string(m-nb+1,'0')+b;
        string ans(m+1,'0');
        int carry=0;
        for(int i=m;i>=0;i--){
            int temp=a[i]-'0'+b[i]-'0'+carry;
            carry=temp/2;
            ans[i]='0'+(temp%2);
        }
        int i=0;
        if(ans[i]=='0') i++;
        return ans.substr(i,m+1-i);
    }
};
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