<p>Given two non-negative integers, <code>num1</code> and <code>num2</code> represented as string, return <em>the sum of</em> <code>num1</code> <em>and</em> <code>num2</code> <em>as a string</em>.</p>

<p>You must solve the problem without using any built-in library for handling large integers (such as <code>BigInteger</code>). You must also not convert the inputs to integers directly.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> num1 = &quot;11&quot;, num2 = &quot;123&quot;
<strong>Output:</strong> &quot;134&quot;
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> num1 = &quot;456&quot;, num2 = &quot;77&quot;
<strong>Output:</strong> &quot;533&quot;
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> num1 = &quot;0&quot;, num2 = &quot;0&quot;
<strong>Output:</strong> &quot;0&quot;
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= num1.length, num2.length &lt;= 10<sup>4</sup></code></li>
	<li><code>num1</code> and <code>num2</code> consist of only digits.</li>
	<li><code>num1</code> and <code>num2</code> don&#39;t have any leading zeros except for the zero itself.</li>
</ul>

class Solution {
public:
    string addStrings(string num1, string num2) {
        int l1=num1.size();
        int l2=num2.size();
        while(l1<l2) {
            l1++;
            num1='0'+num1;
        }
        while(l2<l1) {
            l2++;
            num2='0'+num2;
        }
        string ans="";
        int cry=0;
        for(int i=l1-1;i>=0;i--) {
            int sum=(num1[i]-'0')+(num2[i]-'0');
            sum=sum+cry;
            if(sum>9) {
                cry=sum/10;
                sum=sum%10;
            } else {
                cry=0;
            }
            ans=char(sum+'0')+ans;
        }
        if(cry!=0) {
            ans=char(cry+'0')+ans;
        }
        return ans;
    }
};
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