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

For example,

a = "11"

b = "1"

Return "100".

题意：二进制的加法。

思路：跟大数加法差点儿相同。

class Solution {public:    string addBinary(string a, string b) {        stack
     
       s;        int na = a.size() - 1;        int nb = b.size() - 1;        int c = 0;        while (na > -1 || nb > -1) {            int va = 0;            if (na > -1) {                va = a[na] - '0';                na--;            }            int vb = 0;            if (nb > -1) {                vb = b[nb] - '0';                nb--;            }            int tmp = va + vb + c;            c = tmp / 2;            s.push(tmp % 2);        }        if (c != 0) s.push(c);        string ans;        while (!s.empty()) {            ans.push_back(s.top()+'0');            s.pop();        }        return ans;    }};