<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) {
        string ans="";
        int n1=num1.size()-1,n2=num2.size()-1;
        char carry='0';
        while(n1>=0 && n2>=0){
            int sum=(num1[n1]-'0')+(num2[n2]-'0')+(carry-'0');
            if(sum>9){
                int curr=sum%10;
                ans=char(curr+'0')+ans;
                carry='1';
            }
            else{
                ans=char(sum+'0')+ans;
                carry='0';
            }
            n1--;n2--;
        }
        if(n1>=0){
            while(n1>=0){
                int sum=(num1[n1]-'0')+(carry-'0');
                if(sum>9){
                    int curr=sum%10;
                    ans=char(curr+'0')+ans;
                    carry='1';
                }
                else{
                    ans=char(sum+'0')+ans;
                    carry='0';
                }
                n1--;
            }
        }
        if(n2>=0){
            while(n2>=0){
                int sum=(num2[n2]-'0')+(carry-'0');
                if(sum>9){
                    int curr=sum%10;
                    ans=char(curr+'0')+ans;
                    carry='1';
                }
                else{
                    ans=char(sum+'0')+ans;
                    carry='0';
                }
                n2--;
            }
        }
        if(n1==n2 && n1==-1){
            if(carry!='0'){
                ans=carry+ans;
            }
            // return ans;
        }
        return ans;
    }
};
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