LeetCode::Two Sum C语言

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题目

Given an array of integers, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution.

Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

解答方法一：双重循环尝试，复杂度O(n^2)

#include<stdio.h>

#include<stdlib.h>

int *twoSum(int numbers[], int n, int target)

    if ((numbers == NULL)|(n < 2))

        return NULL;

    int index1;

    int index2;

    int* ret = (int*)malloc(sizeof(int)*2);

    for (index1 = 0; index1 < n; ++index1)

        for (index2 = index1 + 1; index2 < n; ++index2)

            int sum_tmp = numbers[index1] + numbers[index2];

            if (sum_tmp == target)

                ret[0] = index1 + 1;

                ret[1] = index2 + 1;

                return ret;

    return NULL;

int main(int argc, char **argv)

    int array[] = {2, 7, 11, 15};

    size_t n = sizeof(array)/sizeof(int);

    int target = 9;

    int* result = twoSum(array, (int)n, target);

    printf("The index1: %d; the index2: %d\n", result[0], result[1]);

    free(result);//memory deallocation

点评：

解法非常直观，但是复杂度O(n^2)，所以效率很差，在leetcode上无法通过： Submission Result: Time Limit Exceeded

解答方法二：用hash表储存每个数的下标，增加空间使用来加速查找至O(n)

分析：之前O(n^2)的原因是我们做了太多次无谓的运算。事实上，既然我们知道了和和一个参数，只要判断下他们的差（和 - 参数一）存不存在即可。

而用哈希表的话，这个判断存不存在只有O(1)。

下面的代码我是自己实现的hash，所以比C++里用STL代码长很多，但是效率分要比C++高（C++代码有个网友贴了，44ms），而我这个C代码是13ms，大概也是哈希表解法的上限了。

C code

有两个函数 twoSum是解法一； twoSum_hash是解法二

#include<stdio.h>

#include<stdlib.h>

#define PRIME 271

int *twoSum(int numbers[], int n, int target)

    if ((numbers == NULL)|(n < 2))

        return NULL;

    int index1;

    int index2;

    int* ret = (int*)malloc(sizeof(int)*2);

    if (NULL == ret)

        printf("memory allocation for ret failed!\n");

        return NULL;

    for (index1 = 0; index1 < n; ++index1)

        for (index2 = index1 + 1; index2 < n; ++index2)

            int sum_tmp = numbers[index1] + numbers[index2];

            if (sum_tmp == target)

                ret[0] = index1 + 1;

                ret[1] = index2 + 1;

                return ret;

    return NULL;

//second method

typedef struct linkedListNode* plistNode;

typedef struct linkedListNode

    int index;

    plistNode next;

}listNode;

void hashTableFree(plistNode hashTable, int primeNumber)

    int index = 0;

    for (index = 0; index < primeNumber; ++index)

        plistNode tail = hashTable[index].next;

        while(NULL != tail)

            plistNode tmp = tail;

            tail = tail->next;

            free(tmp);

    free(hashTable);

int *twoSum_hash(int numbers[], int n, int target)

    if ((numbers == NULL)|(n < 2))

        return NULL;

    int index = 0;

    //create a hash table

    int primeNumber = PRIME;

    plistNode hashTable = (plistNode)malloc(sizeof(listNode)*primeNumber);

    if (NULL == hashTable)

        printf("memory allocation for hashTable failed!\n");

        return NULL;

    //initialise the hash table

    for (index = 0; index < primeNumber; ++index)

        hashTable[index].index = -1;

        hashTable[index].next = NULL;

    //assign numbers to the hash table

    for (index = 0; index < n; ++index)

        int bias = abs(numbers[index]%primeNumber);

        if (-1 == hashTable[bias].index)

            hashTable[bias].index = index;

        }else //occupied

            plistNode tail = hashTable[bias].next;

            tail = (plistNode )malloc(sizeof(listNode));

            if (NULL == tail)

                printf("memory allocation for tail failed!\n");

                hashTableFree(hashTable, primeNumber);

                return NULL;

            tail->index = index;

            tail->next = hashTable[bias].next;

            hashTable[bias].next = tail;

    //searching

    int* ret = (int*)malloc(sizeof(int)*2);

    for (index = 0; index < n; ++index )

        int bias = abs((target - numbers[index])%primeNumber);

        if (-1 == hashTable[bias].index) // this bias doesn't exist

            continue;

        if ((numbers[hashTable[bias].index] == (target - numbers[index])) & (hashTable[bias].index != index))

            ret[0] = index + 1;

            ret[1] = hashTable[bias].index + 1;

            hashTableFree(hashTable, primeNumber);

            return ret;

        }else

            plistNode tail = hashTable[bias].next;

            while (NULL != tail)

                if (numbers[tail->index] == (target - numbers[index]))

                    ret[0] = index + 1;

                    ret[1] = tail->index + 1;

                    hashTableFree(hashTable, primeNumber);

                    return ret;

                }else

                    tail = tail->next;

    hashTableFree(hashTable, primeNumber);

    return NULL;

int main(int argc, char **argv)

    //int array[] = {438,507,629,255,813,423,536,428,340,767,208,808,882,142,835,423,331,545,627,750,397,675,662,92,465,627,15,522,395,727,561,73,570,606,826,651,743,214,881,685,820,326,653,334,698,604,938,260,51,597,291,855,427,117,943,142,166,439,833,4,180,10,531,350,785,989,607,303,554,764,769,451,654,752,15,90,505,159,1,516,801,938,442,543,761,548,523,766,445,696,217,352,333,383,868,764,556,943,280,140,627,870,635,753,743,978,611,326,405,143,564,256,304,913,570,331,340,222,952,959,535,113,148,125,874,354,984,753,423,448,235,621,796,355,64,682,326,500,609,293,566,974,808,568,729,173,735,764,987,588,227,961,621,340,245,570,640,814,635,482,520,563,695,399,95,92,813,135,342,513,410,943,64,458,801,835,977,932,838,604,500,266,395,108,788,161,769,662,697,167,143,383,880,19,758,552,396,226,548,560,916,766,568,192,100,734,639,288,187,465,345,535,293,130,488,172,108,313,800,662,644,758,843,953,73,543,630,37,711,372,372,410,60,973,750,833,827,572,31,157,473,529,410,650,930,635,253,203,159,431,848,969,982,229,504,617,273,723,854,572,879,382,18,418,990,321,871,544,662,900,340,358,788,998,376,87,520,543,717,67,713,419,108,796,143,342,896,526,926,362,23,63,426,514,526,499,857,87,499,635,496,69,558,635,1,635,125,611,459,11,14,305,969,225,677,354,90,509,317,895,617,798,210,830,63,488,139,500,744,564,295,782,214,569,863,835,14,70,687,321,130,933,822,761,985,637,995,604,932,54,51,170,951,774,117,117,863,940,599,820,473,350,572,776,198,958,1,281,539,874,38,205,591,901,588,743,926,270,675,135,208,588,438,444,752,172,709,191,199,494,743,854,362,340,170,909,434,258,621,983,119,37,281,987,53,486,709,824,817,590,632,798,207,691,592,358,92,316,864,345,488,408,172,392,402,188,880,336,350,739,301,953,314,414,261,701,431,984,350,800,476,370,952,580,585,335,14,486,448,710,637,134,665,166,631,526,877,940,492,855,563,417,517,543,496,979,802,100,100,854,706,350,796,140,46,434,811,543,743,614,838,803,583,280,748,124,570,134,18,282,879,324,142,904,861,884,974,630,319,340,130,954,130,199,681,465,295,597,492,893,552,920,370,221,293,26,632,695,967,372,321,723,907,701,733,520,482,927,11,606,466,404,529,402,166,646,60,802,101,822,93,12,336,136,650,539,640,791,108,997,268,833,562,132,423,879,884,362,374,522,188,496,472,531,434,942,46,528,423,504,664,867,467,685,899,884,746,548,937,296,436,295,486,722,288,465,89,736,900,174,497,37,84,817,411,486,387,949,893,187,776,771,617,175,516,705,296,452,268,6,922,402,991,723,609,98,347,588,554,80,169,197,931,19,597,888,164,347,27,916,378,630,321,644,156,362,4,705,164,465,543,806,282,827,448,670,950,458,388,239,744,507,516,414,459,934,130,924,440,166,788,343,379,884,758,808,912,187,706,419,88,355,996,331,777,500,863,796,627,124,535,561,466,370,101,383,813,44};

    int array[] = {3,2,4};

    size_t n = sizeof(array)/sizeof(int);

    int target = 6;

    int* result = twoSum_hash(array, (int)n, target);

    if (NULL != result)

        printf("The index1: %d; the index2: %d\n", result[0], result[1]);

        printf("values are %d and %d.", array[result[0]-1], array[result[1]-1]);

        free(result);//memory deallocation

    }else

        printf("result is not available!\n");

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