[leetcode]67. Add Binary 二进制加法

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"

注意：

StringBuilder.insert(int offset,  char c) 表示将char放入offset这个偏移量

Character.forDigit(int digit, int radix) 返回radix进制下，该digit所代表的char

代码

class Solution {
    public String addBinary(String a, String b) {
        StringBuilder sb = new StringBuilder(); // new a StringBuilder because it will be easy to append
        
        char[] arrayA = a.toCharArray();// convert char to array 
        char[] arrayB = b.toCharArray();// convert char to array
        
        int i = a.length()-1; 
        int j = b.length()-1; 
        int carry = 0; 
        
        while(i>=0 || j>=0 || carry >0){  
            int updateA = i>=0 ? arrayA[i--]-'0':0; 
            int updateB = j>=0 ? arrayB[j--]-'0':0; 
            int sum = updateA + updateB + carry; 
            sb.insert(0, Character.forDigit(sum%2, 10)); // I convert this sum to binary then insert to sb
            carry = sum/2; // coz it is binary, carry would be sum/2
        } 
        return sb.toString();  //  convert StringBuilder to String
    }
}