Why is the code giving Segmentaation fault?

Question

I was solving the climbing leaderboard problem in hackerrank but my function gives segmentation fault.

vector<int> climbingLeaderboard(vector<int> scores, vector<int> alice) {
    vector<int> res,i;
    auto ip=  unique(scores.begin(),scores.begin()+scores.size());
    scores.resize(distance(scores.begin(),ip));
    for(int i =0;i<alice.size();++i)
    {
    int curr =0;
    while(alice[i]<=scores[curr]&&curr<scores.size())
    ++curr;
    
    if(alice[i]==scores[curr-1])
    res[i]=curr-1;
    else if(alice[i]>scores[curr])
    res[i]=curr;
    else if(curr>scores.size()-1)
    res[i]=curr; 
    
    
    
    }
    
    
    return res;
    
    }

It gives the following error:

> Program terminated with signal SIGSEGV, Segmentation fault.
> #0  0x0000000000402a01 in climbingLeaderboard (scores=..., alice=...)
>     at /usr/local/include/c++/8.3.0/bits/stl_vector.h:930 930       operator[](size_type __n) _GLIBCXX_NOEXCEPT

Answer 1

Firstly, res[i] is accessed without allocating any elements in res.

vector<int> res should be vector<int> res(alice.size()).

Secondly, scores[curr] may be accessed before checking if curr<scores.size().

    while(alice[i]<=scores[curr]&&curr<scores.size())
    ++curr;

should be

    while(curr<scores.size()&&alice[i]<=scores[curr])
    ++curr;

and

    else if(alice[i]>scores[curr])
    res[i]=curr;
    else if(curr>scores.size()-1)
    res[i]=curr; 

should be

    else if(curr>scores.size()-1)
    res[i]=curr; 
    else if(alice[i]>scores[curr])
    res[i]=curr;