leetcode 67 Add Binary ----- java

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

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

 两个二进制数相加，并不难。

public class Solution {
    public String addBinary(String a, String b) {
        int len1 = a.length();
        int len2 = b.length();
        int num = len1>len2?len1:len2;
        char[] result = new char[num];
        int i = len1-1,j = len2-1,flag = 0,k = 0,pos = num-1;
        while( i>=0 && j>=0 ){
            k = a.charAt(i)+b.charAt(j)-'0'-'0'+flag;
            flag = k/2;
            result[pos] = (char) ('0'+k%2);
            i--;
            j--;
            pos--;
        }        
        while( i>=0 ){
            k = a.charAt(i)-'0'+flag;
            flag = k/2;
            result[pos] = (char) ('0'+k%2);
            pos--;
            i--;
        }
        while( j>=0 ){
            k = b.charAt(j)-'0'+flag;
            flag = k/2;
            result[pos] = (char) ('0'+k%2);
            pos--;
            j--;
        }
        if( flag == 1 )
            return "1"+String.valueOf(result);
        else
            return String.valueOf(result);
        
    }
}