Question

我正在解决使用MaxHeap在数组中找到第k个最小元素的问题。我的C代码如下：

#include <stdio.h>
#include <stdlib.h>
/**
 * @input A : Read only ( DON'T MODIFY ) Integer array
 * @input n1 : Integer array's ( A ) length
 * @input k : Integer
 *
 * @Output Integer
 */
struct heap {
     int *A;
     int size;
     int heapsize;
 };

 void init_heap(struct heap*, int);
 void max_heapify(struct heap*, int);
 void add_heap(struct heap*, int);
 int extract_max(struct heap*);
 int get_max(struct heap*);

int kthsmallest(const int* A, int n1, int k) {
    struct heap* H = malloc(sizeof *H);
    init_heap(H, k);
    int i = 0;
    for(i = 0; i < k; i++) {
        add_heap(H, A[i]);
    }
    for(i = k; i < n1; i++) {
        if (A[i] < get_max(H)) {
            extract_max(H);
            add_heap(H, A[i]);
        }
    }
    return get_max(H);
}

//Initializes the heap array, heapsize and size
void init_heap(struct heap* H, int n) {
    H->A = malloc((n+1) * sizeof(int));
    H->heapsize = 0;
    H->size = n;
}

//Makes the tree rooted at index i into a heap if the sub-trees at 2i and 2i+1 are heaps
void max_heapify(struct heap* H, int index) {
    int *arr = H->A;
    int left = 2*index;
    int right = 2*index+1;
    int max = index;
    if (left <= H->heapsize && arr[left] > arr[max]) max = left;
    if (right <= H->heapsize && arr[right] > arr[max]) max = right;
    if (max != index) {
        int temp = arr[index];
        arr[index] = arr[max];
        arr[max] = temp;
    }
    max_heapify(H, max);
}

//Adds and element into the heap
void add_heap(struct heap* H, int data) {
    if(H->heapsize == H->size) {
        return;
    }
    (H->heapsize)++;
    int *arr = H->A;
    arr[H->heapsize] = data;
    int i = H->heapsize;
    while(i > 1 && arr[i/2] < arr[i]) {
        int temp = arr[i];
        arr[i] = arr[i/2];
        arr[i/2] = temp;
        i = i/2;
    }
    //printf("Added %d\n", data);
}

int extract_max(struct heap* H) {
    if (H->heapsize == 0) {
        return -1;
    }
    int *arr = H->A;
    int ret_val = arr[1];
    arr[1] = arr[H->heapsize];
    (H->heapsize)--;
    max_heapify(H, 1);
    //printf("Removed %d\n", ret_val);
    return ret_val;
}

int get_max(struct heap* H) {
    if (H->heapsize == 0) {
        return -1;
    }
    return *((H->A)+1);
}

int main() {
  int A[] = {8, 16, 80, 55, 32, 8, 38, 40, 65, 18, 15, 45, 50, 38, 54, 52, 23, 74, 81, 42, 28, 16, 66, 35, 91, 36, 44, 9, 85, 58, 59, 49, 75, 20, 87, 60, 17, 11, 39, 62, 20, 17, 46, 26, 81, 92};
  printf("Answer = %d\n", kthsmallest(A, 46, 9));
  return 0;
}

当我运行程序时，我遇到了分段错误。我试图调试，但我无法找到原因。

这是我的gdb输出：

Program received signal SIGSEGV, Segmentation fault.
0x0000000100000cd8 in max_heapify (
    H=<error reading variable: Cannot access memory at address 0x7fff5f3ffff8>, index=<error reading variable: Cannot access memory at address 0x7fff5f3ffff4>)
    at kthsmallest.c:49
49  void max_heapify(heap* H, int index) {

这是我的valgrind输出

==21378== Memcheck, a memory error detector
==21378== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==21378== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==21378== Command: ./a.out
==21378==
==21378==
==21378== Process terminating with default action of signal 11 (SIGSEGV)
==21378==  Access not within mapped region at address 0x104002FF8
==21378==    at 0x100000DCE: max_heapify (kthsmallest.c:57)
==21378==  If you believe this happened as a result of a stack
==21378==  overflow in your program's main thread (unlikely but
==21378==  possible), you can try to increase the size of the
==21378==  main thread stack using the --main-stacksize= flag.
==21378==  The main thread stack size used in this run was 8388608.

当 extract_max 函数调用 max_heapify 函数时，会发生细分错误。

Answer 1

这是一个堆栈溢出错误，因为max_heapify继续以递归方式调用自身并且它从未停止过。 max_heapify只应在条件(max != index)为真时递归调用自身。我将递归调用移动到上面的if条件中，代码现在按预期工作。

这是最终正确的功能：

void max_heapify(struct heap* H, int index) {
    int *arr = H->A;
    int left = 2*index;
    int right = 2*index+1;
    int max = index;
    if (left <= H->heapsize && arr[left] > arr[max]) max = left;
    if (right <= H->heapsize && arr[right] > arr[max]) max = right;
    if (max != index) {
        int temp = arr[index];
        arr[index] = arr[max];
        arr[max] = temp;
        max_heapify(H, max);
    }
}

C中函数调用的分段错误

1 个答案: