螺旋矩阵挑战：“ListNode”类型的 null 指针内的成员访问

Question

I am working on LeetCode problem 2326. Spiral Matrix IV :

You are given two integers m and n , which represent the dimensions of a matrix.

You are also given the head of a linked list of integers.

Generate an mxn matrix that contains the integers in the linked list presented in spiral order (clockwise) , starting from the top-left of the matrix. If there are remaining empty spaces, fill them with -1.

Return the generated matrix.

I am trying to solve it using 4 different pointers pointing to the edges of the matrix between which our linked list is travelling and storing the value of the linked list node inside the matrix.

But I get this error:

Line 46: Char 56: runtime error: member access with null pointer of type 'ListNode' (solution.cpp)

SUMMARY: UndefinedBahaviorSanitizer: indefined-behavior prog_joined.cpp:55:46

Here is my code:

class Solution {
public:
    vector<vector<int>> spiralMatrix(int m, int n, ListNode* head) {
        vector<vector<int>>ans(m,vector<int>(n,-1));
        int top=0;
        int bottom=m-1;
        int left=0;
        int right=n-1;
        int dir=0;
        
        while(head){
            switch(dir){
                    
                case 0:{
                    for(int i=left;i<right;i++){
                        ans[top][i]=head->val;
                        head=head->next;
                    }
                    top++;
                    dir=(dir+1)%4;
                    break;
                }
                    
                case 1:{
                    for(int i=top;i<bottom;i++){
                        ans[i][right]=head->val;
                        head=head->next;
                    }
                    right--;
                    dir=(dir+1)%4;
                    break;
                }
                
                case 2:{
                    for(int i=right;i>=left;--i){
                        ans[bottom][i]=head->val;
                        head=head->next;
                    }
                    bottom--;
                    dir=(dir+1)%4;
                    break;
                }
                    
                case 3:{
                    for(int i=bottom;i>=top;--i){
                        ans[i][left]=head->val;
                        head=head->next;
                    }
                    left++;
                    dir=(dir+1)%4;
                    break;   
                }
                    
            }
        }
        return ans;
    }
};

What is causing the error?

Answer 1

Some issues:

• The inner loops do not check whether head is nullptr and risk making an invalid reference with head->next
• As you have defined bottom and right as inclusive (initialising them with m-1 and n-1 ), you would need to align the inner loop condition accordingly with i<=bottom and i<=right .

I would further suggest to not have inner loops, but maintain the current position (row/col) in the matrix and only fill in one value per iteration of the outer loop. This way there is only one test needed for head .

You can also avoid code repetition and use variables to determine which boundary to check and which change to make to the current position (row/col).

Here is how that could look:

class Solution {
public:
    vector<vector<int>> spiralMatrix(int m, int n, ListNode* head) {
        vector<vector<int>> ans(m, vector<int>(n, -1));
        int boundary[4] = {0, n-1, m-1, 0}; // Top, Rightend, Bottom, Leftend
        int direction[4] = {-1, 1, 1, -1}; // Up, Right, Down, Left
        int index[2] = {0, 0}; // Row, Column
        int dir = 1; // Right

        while (head) {
            ans[index[0]][index[1]] = head->val;
            head = head->next;
            if (index[dir % 2] == boundary[dir]) {
                dir = (dir + 1) % 4;
                boundary[dir ^ 2] += direction[dir];
            }
            index[dir % 2] += direction[dir];
        }
        return ans;
    }
};