leetcode[67]Add Binary

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

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

class Solution {
public:
string addBinary(string a, string b) 
{
    int lena=a.size();
    int lenb=b.size();
    if (lena==0)return b;
    if (lenb==0)return a;
    string c="";
    int flag=0;
    while(lena>0&&lenb>0)
    {
        int temp=a[lena-1]-'0'+b[lenb-1]-'0'+flag;
        c=char(temp%2+'0')+c;
        flag=temp/2;
        lena--;
        lenb--;
    }
    while(lena>0)
    {
        int temp=a[lena-1]-'0'+flag;
        c=char(temp%2+'0')+c;
        flag=temp/2;
        lena--;
    }
    while(lenb>0)
    {
        int temp=b[lenb-1]-'0'+flag;
        c=char(temp%2+'0')+c;
        flag=temp/2;
        lenb--;
    }
    if(flag>0)c=char(flag+'0')+c;
    return c;
}
};