<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) {
        if (num1.size() < num2.size()) {
            swap(num1, num2);
        }
        while (num2.size() < num1.size()) {
            num2 = "0" + num2;
        }
        num1 = "0" + num1;
        num2 = "0" + num2;
        string res = string(num1.size(), '0');

        for (int i = res.size() - 1; i >= 1; --i) {
            if ('0' + (res[i] - '0') + (num1[i] - '0') + (num2[i] - '0') > '9') {
                res[i] = '0' + (res[i] - '0') + (num1[i] - '0') + (num2[i] - '0') - 10;
                res[i - 1] = '1';
            } else {
                char curr = '0' + ((num1[i] - '0') + (num2[i] - '0'));
                if (res[i] == '1' && curr == '9') {
                    res[i - 1] = '1';
                    res[i] = '0';
                } else {
                    res[i] = (res[i] - '0') + '0' + ((num1[i] - '0') + (num2[i] - '0'));
                }
            }
        }

        if (res[0] == '0') {
            res = res.substr(1);
        }
        return res;
    }
};
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