Runtime Error. Addition of unsigned offset

Question

I was solving Increasing Subsequences in LeetCode and it is giving error continuously.

Tried in different IDEs with same test cases working every time. Even after brainstorming for an hour or two I was unable to find the error. Here is my C++ solution.

class Solution {
public:
    vector<vector<int>>ans;
    vector<vector<int>> findSubsequences(vector<int>& nums) {
        vector<int>prev;
        calc(nums,prev,0);
        return ans;
    }
     void calc(vector<int>&v, vector<int>&prev, int k)
    {
        if(prev.size()>1)
            ans.push_back(prev);
        if(k<v.size())
        {
            unordered_set<int>s;
            for(int i = k; i < v.size(); i++)
            {
                if((prev.size() == 0 && !s.count(v[i])) || (prev[prev.size()-1]<=v[i] && !s.count(v[i])))
                {
                    prev.push_back(v[i]);
                    calc(v,prev,i+1);
                    s.insert(v[i]);
                    prev.pop_back();
                }
            }
        }
    }
};

Can anyone please help. Showing error in multiple test cases.

Here I am sharing one of the test cases : [4,6,7,7]

Error : Line 1034: Char 34: runtime error: addition of unsigned offset to 0x602000000110 overflowed to 0x60200000010c (stl_vector.h) SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:1043:34

Here is the code which is working in IDEs.

#include <bits/stdc++.h>
using namespace std;
vector<vector<int>>ans;
 void calc(vector<int>&v, vector<int>&prev, int k)
    {
        if(prev.size()>1)
            ans.push_back(prev);
        if(k<v.size())
        {
            unordered_set<int>s;
            for(int i = k; i < v.size(); i++)
            {
                if((prev.size() == 0 && !s.count(v[i])) || (prev[prev.size()-1]<=v[i] && !s.count(v[i])))
                {
                    prev.push_back(v[i]);
                    calc(v,prev,i+1);
                    s.insert(v[i]);
                    prev.pop_back();
                }
            }
        }
    }

int main() {
    vector<int>nums{4,6,7,7};
    vector<int>prev;
    calc(nums,prev,0);
    for(auto i : ans)
    {
        for(auto j : i)
        cout<<j<<" ";
        cout<<endl;
    }
}

it looks odd that `calc` pushes to the member `ans` but it is never cleared. Calling `findSubsequences` twice will probably not produce the right output — 463035818_is_not_an_ai, Aug 11 '22 at 14:15
please post a [mcve]. Code that one can compile and run to reproduce the error. Leetcode uses an ub sanitizer that you can also enable elsewhere — 463035818_is_not_an_ai, Aug 11 '22 at 14:17
*Really no idea how to do that* . You post a `main`, add includes, etc, same you did when you compiled the code with other IDEs. — 463035818_is_not_an_ai, Aug 11 '22 at 14:19
@soumyadip_poddar not related to your problem, but definitely worth reading: https://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h — Jabberwocky, Aug 11 '22 at 14:24

463035818_is_not_an_ai · Accepted Answer · 2022-08-11T16:00:22.477

I was not able to reproduce the santizer message with the input in the question (see https://godbolt.org/z/j4rxGdoYb). However, this condition may wrap around an unsigned integer:

if((prev.size() == 0 && !s.count(v[i])) || (prev[prev.size()-1]<=v[i] && !s.count(v[i])))

When prev.size() == 0 but !s.count(v[i]) is false then prev.size() -1 subtracts 1 from an unsigned 0. As a consequence prev will be accessed out of bounds.

Either use std::ssize to get a signed size. Or make sure that prev[prev.size()-1] is only evaluated when prev.size() != 0.

Moreover, the member ans is never cleared. I expect to get the wrong answer when you call findSubsequences the second time. Also findSubsequences should not take the vector by non-const reference when it does not modify it.

Runtime Error. Addition of unsigned offset

1 Answers1