Why did I get a Memory-related error here?

Question

I had a problem from a website. Given a string s and st , I have to found all possible combination of st in s . For example,

s      = "doomdogged"
st     = "dg"
answer = 4

I can choose the d from 0 or 4, and g from 6 or 7. Which gives me 4 possible combinations.

Here's my code:

#include <iostream>
#include <vector>

using namespace std;

string s, st;

bool target[26];
vector<int> positions[26];
vector<vector<int>> possibleCombinations;

void DFS_Enumeration(int, vector<int>*);
int DFS_index_max = 0;

int main(int argc, char *argv[])
{
    int    answer = 0;
    cin >> s;  //Given a string s
    cin >> st; //Given a string st
    //Find all possible combination of st in s
    for ( int i = 0 ; i < 26 ; ++ i )
        target[i] = 0;
    for ( int i = 0 ; i < st.length() ; ++ i )
        target[st[i] - 97] = 1;
    for ( int i = 0 ; i < 26 ; ++ i )
    {
        if ( target[i] == 0 ) continue;
        for ( int j = 0 ; j < s.length() ; ++ j )
        {
            if ( s[j] == i + 97 ) positions[i].push_back(j);
        }
    }
    DFS_index_max = st.length();
    vector<int> trail(0);
    DFS_Enumeration(0, &trail); //Here I got an runtime error
    for ( vector<int> vi : possibleCombinations )
    {
        int currentMax = 0;
        for ( int i = 0 ; i < vi.size() ; ++ i )
        {
            if ( vi[i] > currentMax )
            {
                if ( i == vi.size() - 1 ) ++ answer;
                currentMax = vi[i];
                continue;
            }
            else
                break;
        }
    }
    cout << answer;
}

void DFS_Enumeration(int index, vector<int>* trail)
{
    if ( index == DFS_index_max )
    {
        possibleCombinations.push_back(*trail);
        return;
    }
    for ( int i = 0 ; i < positions[st[index] - 97].size() ; ++ i )
    {
        trail -> push_back(positions[st[index] - 97][i]);
        DFS_Enumeration(++index, trail);
        trail -> pop_back();
    }
    return;
}

First I look for characters in st , and mark them as needed to found in my boolean array target.

Then, I use DFS to enumerate all possible combinations. For the above example of "doomdogged" and "dg", d exists in 0, 4, 9. And g exist in 6, 7. I will get 06, 07, 46, 47, 96, 97.

Lastly, I count those which make sense, and output the answer. For some reason, my code doesn't work and generate an runtime error concerning memory at the line I've marked.

Answer 1

DFS_Enumeration可以使index增加任意次数，因此st[index]可能超过字符串st的结尾。