Check if a string is a substring of another

Problem

Check if a string is a substring of another string. If true, return the index of the substring’s first letter in the other string. Else, return -1.

Example

input: 
abcde cd
abcde cda
abcd de
abcbcde bc

output:
2
-1
-1
1

written in C++

Solution 1

1. loop through the string a
2. As iterating, check if the current char matches any char of the string b.
3. If they do, iterate through the string b along with a to check if all the chars match
   Time Complexity = O(n)

int substring(string a, string b){
    int result = -1; //result to return
    size_t b_ind = 0; //index to iterate through b
    for(size_t i = 0; i < a.size(); ++i){
        if(b[b_ind] == a[i]){ //if the first char of b matches the char in a
        if (b_ind == 0){ //if it is the first letter in b, save it in result
                result = (int)i;
            }
            if (b_ind == b.size() -1){//if all chars in b are in a, return result
                return result;
            }
            ++b_ind;//update b_ind to iterate
        }else if (b_ind > 0 && a[i-1] == a[i]){ //if b_ind is at 1 and prev char in a equals to the current, cout it as found 
            if (b_ind == 1){                       
                ++result;
            }
        }else{ //if the chars don't match, reset result and b_ind
            if (result != -1){
                result = -1;
            }
            b_ind = 0;
        }

    }

    if (b_ind != b.size()){
        result = -1;
    }
    return result;
}   

Solution 2

Use std::string find(string str)

int substring(string a, string b){
    size_t found = a.find(b);
    if (found == a.npos){
        return -1;
    }else{
        return found;
    }
}

Time Complexity = O(n)

Follow-up

What if the string a contains multiple words with space in between and b is a word?

Example

input: 
"geeks for geeks" "for"
"geeks for geeks" "sfor"

output:
6
-1

Solution is the same with Solution 2.