I'm getting a runtime error for the below code while solving the Pascal's triangle question on Leetcode:

Char 9: runtime error: reference binding to null pointer of type 'int' (stl_vector.h)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior

I did come up with other code that works, but I'd like to know why I'm getting the error for this code.

vector<vector<int>> generate(int numRows) {
    vector<vector<int>> v(numRows);
    vector<int> q;
    for(int i=0;i<numRows;i  ){
        q.resize(i 1);
        q[0]=q[i]=1;
           
        for(int j=1;j<i;j  ){
            q[j]=v[i-1][j-1] v[i-1][j]; 
        }
        v.push_back(q);
        q.clear();
    }
    return v;     
}

CodePudding user response：

You construct vector v with numRows number of elements. And then you iterate from 0 to numRows - 1 and in each iteration, you add a new vector to the and of vector v, so you end up with a vector holding 2 * numRows number of elements instead of numRows number of elements.

Instead of vector<vector<int>> v(numRows); you should use v.reserve(numRows). Here is full code:

vector<vector<int>> generate(int numRows) {
    vector<vector<int>> v;
    v.reserve(numRows);
    
    vector<int> q;

    for(int i=0;i<numRows;i  ){
        q.resize(i 1);
        q[0]=q[i]=1;

        for(int j=1;j<i;j  ){
            q[j]=v[i-1][j-1] v[i-1][j]; 
        }
        v.push_back(q);
        q.clear();
    }

    return v;
}