67. Add Binary.cpp

//Runtime: 4 ms, faster than 100.00% of C++ online submissions for Add Binary.
//Memory Usage: 8.6 MB, less than 61.30% of C++ online submissions for Add Binary.

class Solution {
public:
    string addBinary(string a, string b) {
        int N = max(a.size(), b.size()) + 1;
        
        //padding
        a.insert(a.begin(), N-a.size(), '0');
        b.insert(b.begin(), N-b.size(), '0');
        // cout << a << endl;
        // cout << b << endl;
        //now we can iterate from i = 0
        reverse(a.begin(), a.end());
        reverse(b.begin(), b.end());
        // cout << a << endl;
        // cout << b << endl;
        
        int i = 0, carry = 0;
        //add b to a
        for(int i = 0; i < N; i++){
            int ai = a[i]-'0', bi = b[i]-'0';
            int sumi = ai + bi + carry;
            switch(sumi){
                case 3:
                    a[i] = '1';
                    carry = 1;
                    break;
                case 2:
                    a[i] = '0';
                    carry = 1;
                    break;
                case 1:
                    a[i] = '1';
                    carry = 0;
                    break;
            }
        }
        // cout << a << endl;
        // cout << b << endl;
        
        if(a[N-1] == '0') a.resize(N-1);
        reverse(a.begin(), a.end());
        return a;
    }
};