Add binary string

AddBinary.java

/**
 * LeetCode Add Binary Probem

Given two binary strings, return their sum (also a binary string).
For example,
a = "11"
b = "1"
Return "100".

 */
public String addBinary(String a, String b) {
    int n = a.length(), m = b.length();
    char[] chs = new char[Math.max(n, m) + 1];
    
    int i = n-1, j = m-1, k = chs.length-1, carry = 0;
    while (i >= 0 || j >= 0 || carry > 0){
        int ck = carry;
    	if (i >= 0)
    		ck += a.charAt(i--) - '0';
    	if (j >= 0)
    		ck += b.charAt(j--) - '0';
    	if (ck > 1){
    		ck -= 2;
    		carry = 1;
    	}else
    		carry = 0;
    	chs[k--] = (char)('0' + ck);
    }
    k++;
    while (k < chs.length-1 && chs[k] == '0')
    	k++;
    return new String(chs, k, chs.length - k);
}