LeetCode Q1 二和单遍哈希表不工作

Posted 2023-03-29

【中文标题】LeetCode Q1 二和单遍哈希表不工作

【英文标题】：LeetCode Q1 Two Sum Single Pass Hash Table Not Working

【发布时间】：2022-01-01 14:23:40

【问题描述】：

我正在尝试循环 nums 列表并在哈希表中找到减法，如果没有找到，则将数字插入到哈希表中，直到找到解决方案。但是下面的代码不起作用，有没有人知道我做错了什么？

注意：返回的数组必须是 malloced，假设调用者调用 free()。

int* twoSum(int* nums, int numsSize, int target, int* returnSize)
    
    // create hash table
    // node
    typedef struct node
        int num;
        int index;
        struct node *next;
     node;
    // array
    const unsigned int N = numsSize * 2;
    node *table[N];

    // result
    int *out = malloc(2 * sizeof(int));
     
    // hash function
    unsigned int hash(int num)
        return num * 31337 % N;
    

    // looping the input nums
    for (int i = 0; i < numsSize; i++)

        // calculate compliment
        int compliment = target - nums[i];

        // search for the compliment in linked list
        int compliment_bucket = hash(compliment);
        for (node *n = table[compliment_bucket]; n != NULL; n = n -> next)
            if (compliment == n -> num)
                out[0] = i;
                out[1] = n -> index;
                return out;
            
        

        // if compliment is not in hash table, insert it to the head
        // get the hash value of nums[i]
        int bucket = hash(nums[i]);
        node *tmp = calloc(1, sizeof(node));
        tmp -> num = nums[i];
        tmp -> index = i;
        tmp -> next = NULL;
        if (table[bucket] == NULL)
            table[bucket] = tmp;
        
        else
            tmp -> next = table[bucket];
            table[bucket] = tmp;
        
    

【问题讨论】：

它不起作用是什么意思？顺便说一句，你的代码真的很奇怪，结构和函数是在函数内部定义的。

@kiner_shah 我对 C 编码还很陌生，您介意演示一下如何用 C 编写我的想法吗？

先把所有的结构体和函数定义放到twoSum函数之外。然后看看你得到了什么错误，用这些错误更新这里的帖子（通过复制粘贴以文本格式）。此外，twoSum 似乎不返回int*。

是的，你可以。你所拥有的甚至不是有效的 C。我不确定typedef，但你绝对不能在 C 中拥有函数而不是其他函数

返回一个由两个整数组成的数组@kiner_shah

【参考方案1】：

我将结构和函数移到了外面。另外，请检查// CHANGE HERE标记的cmets。

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

// node
typedef struct node
    int num;
    int index;
    struct node *next;
 node;

// hash function
// CHANGE HERE: pass modulo value to function as argument
unsigned int hash(int num, const unsigned int N)
    // CHANGE HERE: to avoid overflows, multiply with the long value `31337` and then compute modulo
    return (num * 31337l) % N;


// CHANGE HERE: added a function to free the memory (for table)
void freeMemory(node** t, const unsigned int N)

    for (int i = 0; i < N; i++)
    
        node* p = t[i];
        while (p)
        
            node* q = p;
            p = p->next;
            q->next = NULL;
            free(q);
        
    
    free(t);


int* twoSum(int* nums, int numsSize, int target, int* returnSize)
    
    // create hash table
    
    // array
    const unsigned int N = numsSize * 2;
    // CHANGE HERE: use calloc for dynamically allocated arrays
    node **table = calloc(N, sizeof(node*));

    // result
    int *out = calloc(2, sizeof(int));
     
    // looping the input nums
    for (int i = 0; i < numsSize; i++)

        // calculate compliment
        int compliment = target - nums[i];

        // search for the compliment in linked list
        int compliment_bucket = hash(compliment, N);
        for (node *n = table[compliment_bucket]; n != NULL; n = n -> next)
            if (compliment == n -> num)
                out[0] = i;
                out[1] = n -> index;
                // CHANGE HERE: free memory before return
                freeMemory(table, N);
                return out;
            
        

        // if compliment is not in hash table, insert it to the head
        // get the hash value of nums[i]
        int bucket = hash(nums[i], N);
        node *tmp = calloc(1, sizeof(node));
        tmp -> num = nums[i];
        tmp -> index = i;
        tmp -> next = NULL;
        if (table[bucket] == NULL)
            table[bucket] = tmp;
        
        else
            tmp -> next = table[bucket];
            table[bucket] = tmp;
        
    
    // CHANGE HERE: free memory before return
    freeMemory(table, N);
    return out;


int main()

    int size = 2;
    int arr[] = 2, 7, 11, 15;
    int* out = twoSum(arr, 4, 9, &size);
    printf("%d %d\n", out[0], out[1]);