deleting duplicates from a sorted vector - c++

Question

26. Remove Duplicates from Sorted Array
Given a sorted array nums, remove the duplicates in-place such that
each element appear only once and return the new length.
Do not allocate extra space for another array, you must do this by
modifying the input array in-place with O(1) extra memory.
Example 1:
Given nums = [1,1,2],
Your function should return length = 2, with the first two elements of
nums being 1 and 2 respectively.
It doesn't matter what you leave beyond the returned length. Example
2:
Given `nums = [0,0,1,1,1,2,2,3,3,4],
Your function should return length = 5, with the first five elements
of nums being modified to 0, 1, 2, 3, and 4 respectively.
It doesn't matter what values are set beyond the returned length.
Clarification:
Confused why the returned value is an integer but your answer is an
array?
Note that the input array is passed in by reference, which means
modification to the input array will be known to the caller as well.
Internally you can think of this:
// nums is passed in by
reference. (i.e., without making a copy) int len =
removeDuplicates(nums);
// any modification to nums in your function would be known by the
caller. // using the length returned by your function, it prints the
first len elements. for (int i = 0; i < len; i++) {
print(nums[i]); }
i am getting this runtime error while submitting it on leetcode it works fine on coding blocks but shows this error in leetcode compilor
Line 924: Char 9: runtime error: reference binding to null pointer of type 'int' (stl_vector.h)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/stl_vector.h:933:9
class Solution {
public:
int removeDuplicates(vector<int>& nums) {
int k=nums[0];
for(auto i=nums.begin()+1;i<nums.end();i++)
{
if(*i==k) nums.erase(i) , i--;
else k=*i;
}
return nums.size();
}
};
Can anybody help me in finding the cause of error?

Answer 1

Your code works just fine, missing one edge case (an empty nums):
if (nums.empty()) {
return 0;
}
Updated Code:
class Solution {
public:
int removeDuplicates(vector<int>& nums) {
if (nums.empty()) {
return 0;
}
int k = nums[0];
for (auto i = nums.begin() + 1; i < nums.end(); i++) {
if (*i == k) {
nums.erase(i) , i--;
}
else {
k = *i;
}
}
return nums.size();
}
};
Maybe we could just write a simple loop using size(). This'd pass on LeetCode:
// Most of headers are already included;
// Can be removed;
#include <cstdint>
#include <vector>
// Start
static const struct Solution {
using SizeType = std::uint_fast16_t;
static const int removeDuplicates(
std::vector<int>& nums
) {
const SizeType len = std::size(nums);
SizeType count = 0;
for (SizeType i = 1; i < len; ++i) {
if (nums[i] == nums[i - 1]) {
++count;
} else {
nums[i - count] = nums[i];
}
}
return len - count;
}
};