Remove common characters and concatenate.md

Given two strings s1 and s2. Modify both the strings such that all the common characters of s1 and s2 are to be removed and the uncommon characters of s1 and s2 are to be concatenated.
Note: If all characters are removed print -1.

Example 1:

Input: s1 = aacdb s2 = gafd Output: cbgf Explanation: The common characters of s1 and s2 are: a, d. The uncommon characters of s1 and s2 are c, b, g and f. Thus the modified string with uncommon characters concatenated is cbgf.

class Solution
{
    public:
    //Function to remove common characters and concatenate two strings.
    string concatenatedString(string s1, string s2) 
    { 
        unordered_map<char, int> m; 
    	string res = ""; 
    
    	//using map to store all characters of s2 in map.
    	for (int i = 0; i < s2.size(); i++) 
    		m[s2[i]] = 1; 
    
    	//finding characters of s1 that are not present in s2
    	//and appending them to result.
    	for (int i = 0; i < s1.size(); i++) 
    	{ 
    		if (m.find(s1[i]) == m.end()) 
    			res += s1[i]; 
    		else
    			m[s1[i]] = 2; 
    	} 
    
    	//finding characters of s2 that are not present in s1
    	//and appending them to result.
    	for (int i = 0; i < s2.size(); i++) 
    		if (m[s2[i]] == 1) 
    			res += s2[i]; 
    			
    	if(res == "")
    	res = "-1";
    	
    	//returning the result.
    	return res; 
    } 
};