67. Add Binary.md

20. Valid Parentheses

Problem

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

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

tag:

• string
• bit

Solution

二进制加法,注意进位

res = a^b^carry
carry = a+b+carry>1?1:0

java

	public String addBinary(String a, String b) {
		StringBuffer res = new StringBuffer();
		int carry = 0;
		int i, j;
		for (i = a.length() - 1, j = b.length() - 1; i >= 0 && j >= 0; i--, j--) {
			int m = a.charAt(i) - '0', n = b.charAt(j) - '0';
			int r = m ^ n ^ carry;
			carry = (m + n + carry) > 1 ? 1 : 0;
			res.append(r);
		}
		if (i < 0 && j >= 0) {
			while (j >= 0) {
				res.append((b.charAt(j) - '0') ^ carry);
				carry = (b.charAt(j) - '0') & carry;
				j--;
			}
			if (carry == 1)
				res.append(carry);
		} else if (j < 0 && i >= 0) {
			while (i >= 0) {
				res.append((a.charAt(i) - '0') ^ carry);
				carry = (a.charAt(i) - '0') & carry;
				i--;
			}
			if (carry == 1)
				res.append(carry);
		} else {
			if (carry > 0)
				res.append(1);
		}
		return res.reverse().toString();
	}