LeetCode Add Binary

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

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

这题用数组来做可能更简单，但考虑到可能面试的时候要求不能开额外的数组，就只能对string操作了。最主要的是把进位这部分写对。

class Solution {
public:
    string addBinary(string a, string b) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int flag = 0;
        
        int aIndex = a.size() - 1;
        int bIndex = b.size() - 1;
        
        string c;
        
        while(aIndex >= 0 && bIndex >= 0)
        {
            int num = a[aIndex] - '0' + (b[bIndex] - '0') + flag;
            flag = num / 2;
            num %= 2;
            
            c = (char)(num + '0') + c;
            
            aIndex--;
            bIndex--;
        }
        
        while(aIndex >= 0)
        {
            int num = a[aIndex] - '0' + flag;
            flag = num / 2;
            num %= 2;
            
            c = (char)(num + '0') + c;
            
            aIndex--;
        }
        
        while(bIndex >= 0)
        {
            int num = b[bIndex] - '0' + flag;
            flag = num / 2;
            num %= 2;
            
            c = (char)(num + '0') + c;
            
            bIndex--;
        }
        
        if (flag > 0)
            c = (char)(flag + '0') + c;
            
        return c;
    }
};