两个字符串相加的通用解法

N是进制，二进制N = 2, 十进制N = 10

import java.util.*;

public class Solution {

    /**
     * 
     * @param a string字符串 
     * @param b string字符串 
     * @return string字符串
     */
    private static final int N = 2;

    public String addBinary (String a, String b) {
        if (a == null || a.length() == 0) {
            return b;
        }
        if (b == null || b.length() == 0) {
            return a;
        }
        StringBuilder sb = new StringBuilder();
        int carry = 0;
        int i1 = a.length() - 1;
        int i2 = b.length() - 1;
        while (i1 >= 0 || i2 >= 0) {
            int n1 = i1 >= 0 ? a.charAt(i1--) - '0' : 0;
            int n2 = i2 >= 0 ? b.charAt(i2--) - '0' : 0;
            int sum = carry + n1 + n2;
            carry = sum / N;
            sum %= N;
            sb.append(sum);
        }
        if (carry > 0) {
            sb.append(carry);
        }
        return sb.reverse().toString();
    }
}