Add Binary

问题描述

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

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

算法

代码一

public String addBinary(String a, String b) {
        int i = 0, length1 = a.length(), length2 = b.length();
        HashMap<String, String> map = new HashMap<String, String>();
        map.put("000", "00");
        map.put("001", "01");
        map.put("010", "01");
        map.put("011", "10");
        map.put("100", "01");
        map.put("101", "10");
        map.put("110", "10");
        map.put("111", "11");
        String s = "0", result = "", value;
        while (i < length1 && i < length2) {
            s += a.charAt(length1 - 1 - i);
            s += b.charAt(length2 - 1 - i);
            value = map.get(s);
            result += value.charAt(1);
            s = value.substring(0, 1);
            i++;
        }
        if (i >= length1) {
            while (i < length2) {
                s += b.charAt(length2 - 1 - i);
                s += "0";
                value = map.get(s);
                result += value.charAt(1);
                s = value.substring(0, 1);
                i++;
            }
        } else {
            while (i < length1) {
                s += a.charAt(length1 - 1 - i);
                s += "0";
                value = map.get(s);
                result += value.charAt(1);
                s = value.substring(0, 1);
                i++;
            }
        }
        if(s.equals("1"))
            result+=s;
        return new StringBuilder(result).reverse().toString();
    }

代码二

public String addBinary1(String a, String b) {
        StringBuilder sb = new StringBuilder();
        int i = a.length() - 1, j = b.length() - 1, carry = 0;
        while (i >= 0 || j >= 0) {
            int sum = carry;
            if (j >= 0)
                sum += b.charAt(j--) - '0';
            if (i >= 0)
                sum += a.charAt(i--) - '0';
            sb.append(sum % 2);
            carry = sum / 2;
        }
        if (carry != 0)
            sb.append(carry);
        return sb.reverse().toString();

    }

注意事项：

（1）String s="sdhfowf";则s.charAt(0)=='s'。

（2）String变量没有反转函数，StringBuilder有反转函数。