題目

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

用兩個指針分別指向a和b的末尾，將a和b最後一位分別轉為數字，用carry來保存要進位的數字，初始為0。如果a和b相加結束後，carry為1，則在sb之後加一個1，最後將sb逆轉就可得到最終的結果，當然，這裏也是可以用棧來代替reverse()的。

代碼

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();
    }
}
 

[leetcode]67.Add Binary