题目

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

The input strings are both non-empty and contains only characters 1 or 0.

Example 1:

Input: a = "11",b = "1"
Output: "100"
Example 2:

Input: a = "1010",b = "1011"
Output: "10101"

解法

思路

用两个指针分别指向a和b的末尾，将a和b最后一位分别转为数字，用carry来保存要进位的数字，初始为0。如果a和b相加结束后，carry为1，则在sb之后加一个1，最后将sb逆转就可得到最终的结果，当然，这里也是可以用栈来代替reverse()的。

代码

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