LeetCode No67. 二进制求和

class Solution {
    public String addBinary(String a, String b) {
        int len1 = a.length()-1;
        int len2 = b.length()-1;
        int numA = 0;
        int numB = 0;
        int c = 0;
        StringBuffer sb = new StringBuffer();
        while( len1>=0 && len2>=0 ) {
            numA = (int) (a.charAt(len1) - '0');
            numB = (int) (b.charAt(len2) - '0');
            int num = numA + numB + c;
            c = 0;
            if( num>=2 ) {
                num -= 2;
                c = 1;
            }
            len1 --;
            len2 --;
            sb.append(num);
        }
        while( len1>=0 ) {
            numA = (int)(a.charAt(len1) - '0') + c;
            c = 0;
            if( numA>=2 ) {
                numA -= 2;
                c = 1;
            }
            len1 --;
            sb.append(numA);
        }
        while( len2>=0 ) {
            numB = (int)(b.charAt(len2) - '0') + c;
            c = 0;
            if( numB>=2 ) {
                numB -= 2;
                c = 1;
            }
            len2 --;
            sb.append(numB);
        }
        if( c!=0) {
            sb.append(c);
        }
        return sb.reverse().toString();
    }
}