Add Binary

public String addBinary(String a, String b) {
    int x = Integer.parseInt(a, 2);
    int y = Integer.parseInt(b, 2);
    return Integer.toBinaryString(x + y);
}

一开始直接用Integer.toBinaryString()和Integer.parseInt(String, int)方法直接写。小例子还好，可是大例子就过不了，只能强行写了= =

public class Solution {
    public String addBinary(String a, String b) {
        int length = a.length();
        if (b.length() > length) 
            length = b.length();
        String aa = "", bb = "";
        StringBuilder sb = new StringBuilder(length);
        for (int i = 0; i < length - a.length(); ++i)
            aa = aa + "0";
        for (int i = 0; i < length - b.length(); ++i)
            bb = bb + "0";
        aa = aa + a;
        bb = bb + b;
        char last = '0';
        char cha, chb;
        int count = 0;
        for (int i = length - 1; i > -1; i--){
            cha = aa.charAt(i);
            chb = bb.charAt(i);
            count = Character.forDigit(2,last) + Character.forDigit(2,cha) + Character.forDigit(2,chb);
            sb.setCharAt(i, Character.forDigit(2, count % 2));
            last = Character.forDigit(2, count / 2);
        }
        if (last == '0')
            return sb.toString();
        else
            return '1' + sb.toString();
    }
}

注意Character.forDigit是转换char的的方法，可是问题是StringBuilder为空的时候是不能用setCharAt方法的。。。唉
然后还有问题是forDigit写反了 有些还不是用forDigit而是用digit方法

public class Solution {
    public String addBinary(String a, String b) {
    	int length = a.length();
		if (b.length() > length)
			length = b.length();
		String aa = "", bb = "";
		StringBuilder sb = new StringBuilder(length);
		for (int i = 0; i < length; ++i)
            sb.append('0');
		for (int i = 0; i < length - a.length(); ++i)
			aa = aa + "0";
		for (int i = 0; i < length - b.length(); ++i)
			bb = bb + "0";
		aa = aa + a;
		bb = bb + b;
		System.out.println(aa);
		System.out.println(bb);
		char last = '0';
		char cha, chb;
		int count = 0;
		for (int i = length - 1; i > -1; i--) {
			cha = aa.charAt(i);
			chb = bb.charAt(i);
			System.out.println(cha + " " + chb);
			count = Character.digit(last, 2) + Character.digit(cha, 2)
					+ Character.digit(chb, 2);
			System.out.println(count);
			sb.setCharAt(i, Character.forDigit(count % 2, 2));
			last = Character.forDigit(count / 2, 2);
		}
		if (last == '0')
			return sb.toString();
		else
			return '1' + sb.toString();
	}
}

对了Collections.frequency(Collection<T>, T object)方法可以统计某个object出现的次数
上面的代码其实还是出错 "Internal Error" 不知道为什么
下面找了个示例代码 显然要比我写的好很多啊唉。。。

public class Solution {
    public String addBinary(String a, String b) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int l1 = a.length(), l2 = b.length();

        StringBuilder sb = new StringBuilder();
        int carry = 0;

        for (int i = 0; i < Math.max(l1, l2); i++) {
            int b1 = getBitAt(a, l1-1-i), b2 = getBitAt(b, l2-1-i);
            int sum = b1 + b2 + carry;
            sb.append(sum%2);
            carry = sum/2;
        }
        if (carry == 1) {
            sb.append(1);
        }
        return sb.reverse().toString();
    }

    public int getBitAt(String s, int index) {
        if (index < 0) {
            return 0;
        }
        if (s.charAt(index) == '0') {
            return 0;
        }
        else {
            return 1;
        }
    }
}

额 我现在想一下 要是开始把Integer改成Long，说不定就能过了测试了汗。。。当然还是不标准啦~