<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:
    def addStrings(self, num1: str, num2: str) -> str:
        ans = ''
        carry = 0
        a, b = len(num1), len(num2)
        if a < b:
            num1, num2, a, b = num2, num1, b, a
        i = 0
        num1, num2 = num1[::-1], num2[::-1]
        while i < b:
            cur = ord(num1[i]) - 48 + ord(num2[i]) - 48 + carry
            if cur >= 10:
                carry = 1
                cur -= 10
            else:
                carry = 0
            ans += str(cur)
            i += 1
        while carry:
            if i >= a:
                ans += '1'
                break
            else:
                cur = int(num1[i]) + carry
                if cur >= 10:
                    carry = 1
                    cur -= 10
                else:
                    carry = 0
                ans += str(cur)
            i += 1
        else:
            ans += num1[i:]
        return ans[::-1]
        
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