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) {
		swap( a,  b);
		int lenA = a.length();
		int lenB = b.length(); 
		int j = lenB-1; 
		int he, jingwei;
		jingwei = 0; 
		string ans = "";
		for (int i = lenA - 1; i >= 0; i--)
		{
			if (j >= 0){
				he = (a[i] - '0') + (b[j] - '0') + jingwei;
			}
			else   he = (a[i] - '0') + jingwei ; 

			jingwei= he / 2; 
			he = he % 2; 
			ans += to_string(he); 
			j--;
		}
		if (jingwei) ans += '1'; 
		reverse(ans.begin(), ans.end());

		return ans;
	}
	
	
	void swap(string & a, string &  b)
	{
		string temp; 
		if (b.length() > a.length())
		{
			temp = a;
			a = b;
			b = temp;
		}
	}
};