左式堆 , 斜堆 ¶

typedef struct node *LeftistHeap;

typedef struct node {
    int key;
    int npl;
    struct node *left;
    struct node *right;
} Node;

LeftistHeap merge_recursive(LeftistHeap h1, LeftistHeap h2) {
    if (h1 == NULL) return h2;
    if (h2 == NULL) return h1;
    // make sure that h1->key <= h2->key always holds
    if (h1->key > h2->key) {
        LeftistHeap temp = h1;
        h1 = h2;
        h2 = temp;
    }
    h1->right = merge_recursive(h1->right, h2);
    /* swap left and right if necessary
    in case that the left child is NULL */
    if (h1->left == NULL) {
        h1->left = h1->right;
        h1->right = NULL;
    } else {
        if (h1->left->npl < h1->right->npl) {
            LeftistHeap temp = h1->left;
            h1->left = h1->right;
            h1->right = temp;
        }
        h1->npl = h1->right->npl + 1;
    }
    return h1;
}

void npl_update(LeftistHeap h) {
    if (h == NULL) return;
    h->npl = h->right == NULL ? 0 : h->right->npl + 1;
}

LeftistHeap merge_iterative(LeftistHeap h1, LeftistHeap h2) {
    if (h1 == NULL) return h2;
    if (h2 == NULL) return h1;
    LeftistHeap stack[MAX_STACK_SIZE] = {NULL};
    int top = -1;
    LeftistHeap h = NULL;
    LeftistHeap *p = &h;
    while (h1 != NULL && h2 != NULL) {
        if (h1->key > h2->key) {
            LeftistHeap temp = h1;
            h1 = h2;
            h2 = temp;
        }
        stack[++top] = h1;
        *p = h1;
        p = &h1->right;
        LeftistHeap next = h1->right;
        h1->right = h2;
        h1 = next;
    }
    *p = h1 == NULL ? h2 : h1;
    while (top >= 0) {
        npl_update(stack[top--]);
    }
    return h;
}