原题链接: https://oj.leetcode.com/problems/add-binary/

public String addBinary(String a, String b) {
    if (a == null && b == null) return null;

    StringBuilder sb = new StringBuilder(a);

    int i = sb.length() - 1;
    int j = b.length() - 1;
    int carry = 0;
    char ac;
    char bc;

    while (i >= 0 && j >= 0) {
        ac = sb.charAt(i);
        bc = b.charAt(j);

        sb.setCharAt(i, (char) ('0' + (ac - '0' + bc - '0' + carry) % 2));
        carry = (ac - '0' + bc - '0' + carry) / 2;

        i--;
        j--;
    }

    while (i >= 0) {
        ac = sb.charAt(i);
        sb.setCharAt(i, (char) ('0' + (ac - '0' + carry) % 2));
        carry = (ac - '0' + carry) / 2;
        i--;
    }

    while (j >= 0) {
        bc = b.charAt(j);
        sb.insert(0, (char) ('0' + (bc - '0' + carry) % 2));
        carry = (bc - '0' + carry) / 2;
        j--;
    }

    if (carry > 0) {
        sb.insert(0, '1');
    }

    return sb.toString();
}