<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 len1 = num1.length() , len2 = num2.length();
        if(len2>len1) for(int i = 0 ; i < len2-len1;i++) num1 = '0' + num1;
        else if(len1>len2) for(int i = 0 ; i < len1-len2;i++) num2 = '0' + num2;
        int len = num1.length();
        int sum=0,carry=0;
        for(int i=len-1;i>-1;i--){
            sum = (num1.at(i) - '0') + (num2.at(i)-'0') + carry;
            carry = sum/10;
            sum %= 10;
            num1.at(i) = sum+'0';
        }
        if(carry) num1 = '1' + num1;
        return num1;
    }
};
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