I’m now solving the following problem in virtualjudge, related with Kd-tree.
https://vjudge.net/problem/Aizu-DSL_2_C#google_vignette

By the way, a runtime error occurred in the code I wrote, even though it passed the test case on the local computer.

This program is written by C. And I guess this issue occurs in the createKdTree() method. How do I solve it?

* my code’s part *

typedef struct {
    int x, y;
} Point;

typedef struct Node {
    int index;
    struct Node *left, *right;
} node;

Point points[MAX_N];
int sortedX[MAX_N][2], sortedY[MAX_N][2];

...

node *createNode(int index) {
    node *newNode = (node *) malloc(sizeof(node));
    if (newNode == NULL) {
        fprintf(stderr, "Failed to allocate memory for new node.n");
        exit(EXIT_FAILURE);
    }
    newNode->index = index;
    newNode->left = newNode->right = NULL;
    return newNode;
}

node *createKdTree(int l, int r, bool byX, bool isfirst) {
    if (l > r) return NULL;
    if (l == r) return createNode(sortedX[l][0]);
    
    int m = (l + r) / 2;
    int slice = m;
    node *root;

    if (isfirst) {
        if (byX) {
            while (slice < r-1 && points[sortedX[slice][0]].x == points[sortedX[m][0]].x) slice++;
        } else {
            while (slice < r-1 && points[sortedY[slice][0]].y == points[sortedY[m][0]].y) slice++;
        }

        root = createNode(m);
        root->left = createKdTree(l, slice-1, !byX, false);
        root->right = createKdTree(slice+1, r, !byX, false);
        return root;
    }
    else {
        int k = transmit(byX, m);
        int i = transmit(byX, slice);
        if (byX) {
            while (slice < r-1 && points[sortedY[i][0]].x == points[sortedY[k][0]].x) i = transmit(byX, ++slice);
        } else {
            while (slice < r-1 && points[sortedX[i][0]].y == points[sortedX[k][0]].y) i = transmit(byX, ++slice);
        }

        root = createNode(k);
        root->left = createKdTree(l, slice-1, !byX, false);
        root->right = createKdTree(slice+1, r, !byX, false);
        return root;
    }