LeetCode #389 Find the Difference Solution & Explanation

public class Solution {
    public char FindTheDifference(string s, string t) {
        
        Dictionary<char,int> dict =   new Dictionary<char,int>();

        //1.count number of occurrences of each value
        foreach(var tmp in s)
        {
            if (!dict.ContainsKey(tmp))
            {
                dict.Add(tmp, 1);
            }
            else
            {
                dict[tmp] = dict[tmp]+1;
            }
        }

        //2. find the added char
        foreach(var tmp in t){
            if (!dict.ContainsKey(tmp)){
                return tmp;
            }
            var num = dict[tmp]-1;
            if(num<0){
                return tmp;
            }
            dict[tmp] = num;
        }

        return '\0';    // Default value used to ensure every possible code path returns a value.
    }
}

class Solution {
    public char findTheDifference(String s, String t) {
        Map<Character, Integer> dict =  new HashMap<Character, Integer>();

        //1.count number of occurrences of each value
        for (char tmp: s.toCharArray()) 
        {
            if (!dict.containsKey(tmp))
            {
                dict.put(tmp, 1);
            }
            else
            {
                dict.put(tmp, dict.get(tmp)+1);
            }
        }

        //2. find the added char
        for (char tmp: t.toCharArray()) {
            if (!dict.containsKey(tmp)){
                return tmp;
            }
            int num = dict.get(tmp)-1;
            if(num<0){
                return tmp;
            }
            dict.put(tmp,num);
        }

        return '\0';    // Default value used to ensure every possible code path returns a value.
    }
}