What is int** returnColumnSizes in leetcode problem 46 (language: c)

Question

This question is based around leetcode problem 46: Permutations.

copy of the question for convenience: Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order.

Constraints:

1 <= nums.length <= 6 -10 <= nums[i] <= 10 All the integers of nums are unique.

I am a beginner in C and i'm trying to work on my pointer and malloc skills. I don't have much experience on any of this, so this question is difficult for me.

Leetcode gives a set input you cannot change, and the last input is giving me some trouble. This is the function. int** permute(int* nums, int numsSize, int* returnSize, int** returnColumnSizes)

I have found a 'solution' to this problem that I checked on examples in VS code. This is the solution.

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>

/**
 * Return an array of arrays of size *returnSize.
 * The sizes of the arrays are returned as *returnColumnSizes array.
 * Note: Both returned array and *columnSizes array must be malloced, assume caller calls free().
 */

/*
numsSize is max 6.
*/
void backTrack(int** res, int* nums, int numsSize, int* permutation, int permutationSize, bool* used, int* numberadded);

int** permute(int* nums, int numsSize, int* returnSize, int** returnColumnSizes) {

    // Determine resturnSize
    int factorialLookUpTable[6] = {1,2,6,24,120,720};
    *returnSize = factorialLookUpTable[numsSize-1];

    // Malloc
    int **returnedArray = malloc(sizeof(int*)* (*returnSize)); // An array that hold 'returnSize' pointers.

    // Inititalizing the arrays inside returnedArray
    for(int i = 0; i< *returnSize; i++){
        returnedArray[i] = malloc(sizeof(int)* numsSize);
    }

    int* permutation = malloc(numsSize*sizeof(int));
    bool* used = calloc(numsSize, sizeof(bool));
    int numberAdded = 0;

    backTrack(returnedArray, nums, numsSize, permutation, 0, used, &numberAdded);

    return returnedArray;
    
}

void backTrack(int** res, int* nums, int numsSize, int* permutation, int permutationSize, bool* used, int* numberadded){


    if (permutationSize == numsSize){
        // Add the permutation to the result.
        memcpy(res[*numberadded],permutation,numsSize*sizeof(int));
        *numberadded += 1;
        return;

    }

    // Go through all digits and combinations.
    for (int i = 0; i < numsSize; i++){
        if (! used[i]){

            used[i] = true;
            permutation[permutationSize] = nums[i];
            permutationSize++;

            backTrack(res, nums, numsSize, permutation, permutationSize, used, numberadded); // recursion

            used[i] = false;
            permutation[permutationSize-1] = 0;
            permutationSize--;

        }
        
    }

}

The biggest issue for me is setting up 'res' and storing data in it so it can be extracted in a simple way. Now this solution 'works' (at least my examples do) but leetcode gives the error: "Line 241: Char 15: runtime error: load of null pointer of type 'int' [Serializer.c]".

My guess is that i had to use returnColumnSizes somewhere, but i have no idea what it means (because it's a pointer to a pointer). Could anyone give me some suggestion on how to improve on this code and how it's ment to be solved? Thanks

Have you worked through a C textbook, primer, or course? That is a better way to learn C than attempting online challenges you are not prepared for. — Eric Postpischil
– Eric Postpischil, Commented Sep 11, 2024 at 12:55
I will, i just wanted to get a feeling for the language. I have done other problems that weren't really a problem at all. As a small follow up question, when would I be prepared for this? — Tibetje2
– Tibetje2, Commented Sep 11, 2024 at 13:24

pmg · Accepted Answer · 2024-09-11 13:05:30Z

2

What is int** returnColumnSizes?

The calling function (which you do not have access to) will do something like

void checkAnswer(void) {
    int values[] = {5, 3, -1, 0, -2, 8};
    int rows, *cols;

    int **y = permute(values, 6, &rows, &cols);
    for (int row = 0; row < rows; row++) {
        for (int col = 0; col < cols[row]; col++) {
            // check y[row][col]
       }
   }
   // free stuff
}

answered Sep 11, 2024 at 13:05

pmg

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3 Comments

Tibetje2 Over a year ago

This helped alot for me, i managed to get it to work. Is there an underlying reason that we don't give a single pointer for cols? My intuition says that it's because of the yet unknown size of cols. Would you agree on that or is there something else going on?

pmg Over a year ago

cols can vary for each row, ie, the 1st row can have 2 columns, the 2nd row can have 5 columns, etc...

pmg Over a year ago

... well ... lol.... different columns for different rows is not a thing here, but leetcode is maybe preparing for a future problem where it will be significant :-)

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