Add binary

Given two binary strings a and b, return their sum as a binary string.

Example 1:
Input: a = "11", b = "1"
Output: "100"

Example 2:
Input: a = "1010", b = "1011"
Output: "10101"
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// n=a.length(), m=b.length() tc=O(n+m) sc=O(n+m)
class Solution {
public:
    string addBinary(string a, string b) {
        string res="";
        int i=a.size()-1, j=b.size()-1;
        int carry=0;
        while(i>=0 || j>=0 || carry){
            int sum=carry;
            
            if(i>=0){
                sum+=a[i]-'0';
                i--;
            }

            if(j>=0){
                sum+=b[j]-'0';
                j--;
            }

            res+=(sum%2)+'0';
            carry=sum/2;
        }
        reverse(res.begin(), res.end());
        return res;
    }
};