reference binding to null pointer of type 'value_type'

Question

This is leetcode 26. Given a sorted array, remove the duplicates in-place such that each element appear only once and return the new length. An example is given nums = [1,1,2], the function should return [1,2].

Below is my code. I delete all the other duplicates, just leave one of them. However I always got an error of reference binding to null pointer of type 'value_type' when submitting. I would appreciate if anyone can help me with this!

class Solution {
public:
    int removeDuplicates(vector<int>& nums) {
        int i = 0;
        while(i < nums.size() - 1) {
            if (nums[i] == nums[i + 1]) {
                nums.erase(nums.begin() + i);
            } 
            else i++;
        }
        return nums.size();
    }
};

Can't duplicate. ideone.com/ppuRg5.

R Sahu
– R Sahu

2017-12-22 22:25:23 +00:00
Commented Dec 22, 2017 at 22:25 — R Sahu
– R Sahu, Commented Dec 22, 2017 at 22:25
Show a complete program that gives the error message.

Jive Dadson
– Jive Dadson

2017-12-22 22:29:53 +00:00
Commented Dec 22, 2017 at 22:29 — Jive Dadson
– Jive Dadson, Commented Dec 22, 2017 at 22:29
@RSahu Good example of a lacking test suite ;)

Lightness Races in Orbit
– Lightness Races in Orbit

2019-01-28 15:20:29 +00:00
Commented Jan 28, 2019 at 15:20 — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jan 28, 2019 at 15:20

David G · Accepted Answer · 2017-12-22 22:30:33Z

46

vector<T>::size() returns a value of type size_t, which is an unsigned type. Let's say the vector passed in is empty and therefore the vector's length is 0. nums.size() - 1 will cause integer underflow and you will actually be comparing 0 with a very large positive number. This will evaluate to true causing the loop to run and i going pass the array bounds.

To fix this you can cast nums.size() to int preemptively or store the size in an integer variable and compare with that.

answered Dec 22, 2017 at 22:30

David G

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1 Comment

Barmar Over a year ago

Or change it to while (i + 1 < nums.size()). This will work as long as nums.size() is not INT_MAX.

R Sahu · Accepted Answer · 2017-12-22 22:30:59Z

The function, as posted, works fine for a vector whose elements are [1 1 2]. See https://ideone.com/ppuRg5.

However, one problem I see in your function is that if you pass it an empty vector, it's going to run into problems.

while(i < nums.size() - 1)

will be a problem when nums is empty. You can preemptively avoid that problem by returning from the function immediately if you find that it is an empty vector.

Also, use an unsigned type for i to avoid compiler warnings about comparing signed and unsigned types.

int removeDuplicates(std::vector<int>& nums) {
   if ( nums.empty() )
   {
      return 0;
   }

   unsigned int i = 0;
   while(i < nums.size() - 1) {
      if (nums[i] == nums[i + 1]) {
         nums.erase(nums.begin() + i);
      } 
      else i++;
   }
   return nums.size();
}

Rain Forest · Accepted Answer · 2017-12-22 22:41:22Z

2

This is not an answer to your question, but it would be more efficient solution to the problem if you didn't have to resize your vector each time you find a duplicate. Just to give you an idea, you could have two iterators i and j, i keeping the index of the last unique element of your solution vector and j iterating through the vector. When j points to a value not in the first i elements, copy that to v[i]. And once you are done, delete everything from the j-th place onwards.

answered Dec 22, 2017 at 22:41

Rain Forest

663 bronze badges

Comments

alexmosk25 · Accepted Answer · 2019-02-25 12:58:22Z

0

In my case the reason of this error message was segmentation fault.

Example For empty input:

string longestCommonPrefix(vector<string>& strs) { 
    auto start = strs[0];

It works OK, when I add check for empty input

string longestCommonPrefix(vector<string>& strs) {
        if (strs.size() == 0) {
            return "";
        }
    auto start = strs[0];

answered Feb 25, 2019 at 12:58

alexmosk25

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Comments

slfan · Accepted Answer · 2019-11-26 10:17:58Z

0

class Solution {
  public:
    int removeDuplicates(vector < int > & nums) {
        for (int i = 1; i < nums.size(); i++) {
            if (nums[i] == nums[i - 1]) {
                nums.erase(nums.begin() + i);
                i--;
            }
        }
        return nums.size();
    }
};

edited Nov 26, 2019 at 10:17

slfan

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answered Nov 26, 2019 at 9:36

ansar

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Comments

Simas Joneliunas · Accepted Answer · 2020-08-03 00:09:57Z

-2

I just figured out that instead of comparing or operating num.size() with any integer directly First store int n=num.size(); and then compare it like if(num=n-2).

edited Aug 3, 2020 at 0:09

Simas Joneliunas

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answered Aug 2, 2020 at 20:10

Sandy

1

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