Create 67. Add Binary.md

67. Add Binary.md

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+# [67. Add Binary](https://leetcode.com/problems/add-binary/description/)
+# 思路
+题目要求实现多位二进制加法器。可以先考虑先实现一个一位二进制加法器，然后再运用这个一位二进制加法串接成多位的二进制加法，注意不要忘了处理进位。  
+注意学习如何将char型的vector转成string: `string str(vc.begin(), vc.end())`
+# C++
+```
+class Solution {
+private:
+    int bit_add(const int a, const int b, int &cin){ // 一位加法器，cin是进位
+        int tmp = a + b + cin;
+        cin = tmp / 2;
+        return tmp % 2;
+    }
+public:
+    string addBinary(string a, string b){
+        vector<char>res;
+        int high1  = a.size() - 1, high2 = b.size() - 1, cin = 0, sum;
+        while(high1 >= 0 && high2 >= 0){
+            sum = bit_add(a[high1--] - '0', b[high2--] - '0', cin);
+            res.push_back(sum + '0');
+        }
+        while(high1 >= 0){
+            sum = bit_add(a[high1--] - '0', 0, cin);
+            res.push_back(sum + '0');
+        }
+        while(high2 >= 0){
+            sum = bit_add(b[high2--] - '0', 0, cin);
+            res.push_back(sum + '0');
+        }
+        if(cin == 1) res.push_back('1');
+        reverse(res.begin(), res.end());
+        return string(res.begin(), res.end());
+    }
+};
+```