leetcode--Add Binary

Given two binary strings, return their sum (also a binary string).

For example,
a = "11"
b = "1"
Return "100".

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class Solution {
public:
    string addBinary(string a, string b) 
    {
        int lena = a.size();
        int lenb = b.size();
        int i,j;
        if(lena==0||lenb==0)return a+b;
        string str;
        int carry = 0;
        int tem;
        for(i=lena-1, j=lenb-1;i>=0&&j>=0;i--,j--)
        {
            tem   = (a[i]+b[j]+carry-2*'0')%2;
            carry = (a[i]+b[j]+carry-2*'0')/2;
            str = (char)(tem + '0') + str;
            
        }
        if(i>=0)
        {
            while(i!=-1)
            {  
               
                tem   = (a[i]+carry-'0')%2;
                carry = (a[i]+carry-'0')/2;
                str = (char)(tem + '0') + str;
                 i--;
            }
        }
        if(j>=0)
        {
            while(j!=-1)
            {
               
            tem   = (b[j]+carry-'0')%2;
            carry = (b[j]+carry-'0')/2;
            str = (char)(tem + '0') + str;
             j--;
            }
        }
        if(carry==1)
            str = '1'+str;
        return str;
    }
};