leetcode Question 4: Add binary

Add binary:

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

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

Analysis:

This is an easy problem but needs carefulness.
Main idea has 2 steps:
1. Check the length of each string and add "0" to the shorter one.
2. Sum up the two strings, using a "carry" variable to store the carry. 

Note that in Python, one can not assign value for specific char in a string, the list() operation is needed to change string into list of chars.

Code(C++):

class Solution {
public:
    string addBinary(string a, string b) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        string res = a.size()>b.size()?a:b;
        string str = a.size()>b.size()?b:a;
        bool carry=false;
        int i=str.size()-1;    
        int j=res.size()-1;
        while (i>=0){
            if (res[j]=='1' && str[i]=='1'){res[j]=carry?'1':'0';carry=true;i--;j--;continue;}
            if (res[j]=='0' && str[i]=='0'){res[j]=carry?'1':'0';carry=false;i--;j--;continue;}
            if (res[j]=='0' && str[i]=='1'){res[j]=carry?'0':'1';i--;j--;continue;}
            if (res[j]=='1' && str[i]=='0'){res[j]=carry?'0':'1';i--;j--;continue;}
        }
        while (j>=0 && carry){
            if (res[j]=='1'){res[j]='0';j--;continue;}
            if (res[j]=='0'){res[j]='1';break;}
        }
        if (j<0 && carry){ res="1"+res;}
        return res;
    }
};

Code(Python):

class Solution:
    # @param a, a string
    # @param b, a string
    # @return a string
    def addBinary(self, a, b):
        if len(a) > len(b):
            b = b.rjust(len(a), '0')
        else:
            a = a.rjust(len(b), '0')
        a = list(a)
        b = list(b)
        carry = '0'
        for i in xrange(len(a)-1,-1,-1):
            if a[i] == '0' and b[i] == '0':
                a[i] = carry
                carry = '0'
            
            elif a[i] == '1' and b[i] == '0':
                if carry == '1':
                    a[i] = '0'
                    carry = '1'
              
            elif a[i] == '0' and b[i] == '1':
                if carry == '1':
                    a[i] = '0'
                    carry = '1'
                else:
                    a[i] = '1'
              
            elif a[i] == '1' and b[i] == '1':
                if carry == '1':
                    a[i] = '1'
                else:
                    a[i] = '0'
                    carry = '1'
                
        if carry == '1':
            a.insert(0, '1')
            
        return ''.join(a)