LintCode_408 二进制求和

string addBinary(string& a, string& b) {
	// Write your code here
    	int i = a.size() - 1;
    	int j = b.size() - 1;
    	
    	int cp = 0;
    	string s = "";
    	while (i >= 0 || j >= 0)
    	{
    		char a1;
    		char b1;
    		if (i >= 0) a1 = a[i];
    		else a1 = ‘0‘;
    		if (j >= 0) b1 = b[j];
    		else b1 = ‘0‘;
    		int p = (a1 - ‘0‘) + (b1 - ‘0‘) + cp;
    		if (p == 0)
    		{
    			s = ‘0‘ + s;
    			cp = 0;
    		}
    		else if (p == 1)
    		{
    			s = ‘1‘ + s;
    			cp = 0;
    		}
    		else if (p == 2)
    		{
    			s = ‘0‘ + s;
    			cp = 1;
    		}
    		else if (p == 3) {
    			s = ‘1‘ + s;
    			cp = 1;
    		}
    		if(i != -1) i--;
    		if(j != -1) j--;
    	}
    	
    	if (cp == 1)
    	{
    		s = ‘1‘ + s;
    	}
    	return s;
    }