leetcode Add Binary

class Solution {
public:
    string addBinary(string a, string b) {
        if(a.size()==0 || b.size()==0) return a+b;
        string ans = "";
        int up = 0, lena = a.size() - 1, lenb = b.size() - 1;
        while(lena >= 0 && lenb >= 0)
        {
            if (a[lena] == b[lenb] && a[lena] == ‘1‘)
            {
                if(up == 1)
                    ans = ‘1‘ + ans;
                else
                {
                    ans = ‘0‘ + ans; up=1;         
                }
            }
            else if (a[lena] == ‘0‘ && b[lenb] == ‘0‘)
            {
                if(up == 1)
                    {ans = ‘1‘ + ans; up = 0;}
                else
                    ans = ‘0‘ + ans;
            }
            else
            {
                if(up == 1)
                    {ans = ‘0‘ + ans; up = 1;}
                else
                    ans = ‘1‘+ ans;
            }
            lena--; lenb--;
        }
        while(lena>=0)
        {
            if(a[lena] == ‘1‘ && up == 1)
                ans = ‘0‘ + ans;
            else if (up == 1)
            {
                ans = ‘1‘ + ans; up = 0;
            }
            else
            {
                ans = a[lena] + ans;
            }
            lena--;
        }
        while(lenb>=0)
        {
            if(b[lenb] == ‘1‘ && up == 1)
                ans = ‘0‘ + ans;
            else if (up == 1)
            {
                ans = ‘1‘ + ans; up = 0;
            }
            else
            {
                ans = b[lenb] + ans;
            }
            lenb--;
        }
        if(up == 1)
            ans = ‘1‘ + ans;
        return ans;
    }
};