[LeetCode]Add Binary

给定字符串a,b，分别表示两个二进制数。将a,b的二进制和以字符串形式返回。

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

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

思路：模拟二进制加法过程，从最低位开始

难点在于进位的计算。记进位符为carry, 该位的和为num

int num = a[i]-'0'+b[j]-'0'+carry;
carry = num/2;
num %= 2;

代码

class Solution {
public:
    string addBinary(string a, string b) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        string ret;
        int i=a.length()-1, j=b.length()-1;
        int carry = 0;
        while(i>=0 && j>=0)
        {
            int num = a[i]-'0'+b[j]-'0'+carry;
            carry = num/2;
            num %= 2;
            ret = char(num+'0') + ret;
            --i; --j;
        }
        // add carry and remaining string
        while(i>=0)
        {
            int num = a[i]-'0'+carry;
            carry = num/2;
            num %=2;
            ret = char(num+'0')+ret;
            --i;
        }
        while(j>=0)
        {
            int num = b[j]-'0'+carry;
            carry = num/2;
            num %=2;
            ret = char(num+'0')+ret;
            --j;
        }
        
        if(carry>0)
            ret = "1"+ret;
        
        return ret;
    }
};