Is there any O(n) solution to solve this. In c++.

public static String twoStrings(String s1, String s2) {
    String maxstring = s1.length() > s2.length() ? s1 : s2;
    String smallstring = s1.length() <= s2.length() ? s1 : s2;
    Set<String> maxset = Arrays.stream(maxstring.split("")).collect(Collectors.toSet());
    Set<String> smallset = Arrays.stream(smallstring.split("")).collect(Collectors.toSet());
    int smallsetsize = smallset.size();
    smallset.removeAll(maxset);
    return smallsetsize == smallset.size() ? "NO" : "YES";
}

python O(n + m) time O(n) space:

def twoStrings(s1, s2):
    strset = set(s1)
    
    for c in s2:
        if c in strset:
            return "YES"
    
    return "NO"

Python3

def twoStrings(s1, s2):
    int_set = set(s1).intersection(set(s2))
    return 'YES' if len(int_set)>0 else 'NO'

Here is my simple C++ solution:

string twoStrings(string s1, string s2) {
    
    map<char,int> m1;
    for(int i=0;i<s1.size();i++){
        m1[s1[i]]++;
    }
    for(int i=0;i<s2.size();i++){
        if(m1[s2[i]]){
            return "YES";
        }
    }
    return "NO";
}