暴力算法：两层遍历循环，第一层找出一个i，第二层找出j，如果i+j=target，则返回；复杂度为O(n^2)。
哈希表算法：先将数组各数及其索引绑定插入哈希表中，从循环遍历数组nums[i]，看target-nums[i]是否在哈希表中，如果哈希表中存在nums[j]==target-nums[i]，则nums[i]+nums[j]==target，返回(i,j)；遍历数组的复杂度为O(n)，从哈希表中查找元素的复杂度为O(1)，则总复杂度为O(n)*O(1)=O(n)。

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <windows.h>//结点定义
typedef struct Node
{int index;int num;struct Node *next;
}Node;
//哈希表定义
typedef struct HashTable
{Node **B;   //Node结点的指针数组，存储Node结点的指针int len;    //数组长度
}HashTable;void Init_HT(HashTable *ht,int n);
void Free_HT(HashTable *ht);
void Insert_Item_HT(HashTable *ht,int index,int num);
int Search_Item_HT(HashTable *ht,int num);
int *twosum(int *nums,int n,int target,int *returnSize);    //哈希表算法：复杂度O(n)
int *twosums(int *nums,int n,int target,int *returnSizes);  //暴力算法：复杂度O(n^2)
int main()
{int *nums;int n;printf("输入数组数量：");scanf("%d",&n);nums=(int *)malloc(sizeof(int)*n);//利用随机函数生成数组int i;for(i=0;i<n;i++)nums[i]=rand()+i;int target,*returnSize,*returnSizes;returnSize=(int *)malloc(sizeof(int));returnSizes=(int *)malloc(sizeof(int));target=nums[i-1]+nums[i-2];printf("\n哈希表算法：\n");int *result;DWORD t1,t2;t1=GetTickCount();result=twosum(nums,n,target,returnSize);t2=GetTickCount();if(*returnSize==2)printf("nums[%d]:%d + nums[%d]:%d = %d\nt1:%d t2:%d \ntimeconsuming:%d\n",result[0],nums[result[0]],result[1],nums[result[1]],target,t1,t2,t2-t1);elseprintf("无！");printf("\n暴力算法：\n");int *result2;DWORD t3,t4;t3=GetTickCount();result2=twosums(nums,n,target,returnSizes);t4=GetTickCount();if(*returnSizes==2)printf("nums[%d]:%d + nums[%d]:%d = %d\n t3:%d t4:%d \ntimeconsuming:%d\n",result2[0],nums[result2[0]],result2[1],nums[result2[1]],target,t3,t4,t4-t3);elseprintf("无！");return 0;
}
//暴力算法：复杂度O(n^2)
int *twosums(int *nums,int n,int target,int *returnSizes)
{int i,j,*k;k=(int *)malloc(sizeof(int)*2);for(i=0;i<n-1;i++)for(j=i+1;j<n;j++){if(nums[i]+nums[j]==target){k[0]=i;k[1]=j;*returnSizes=2;return k;}}*returnSizes=0;return NULL;
}
//哈希表算法：复杂度O(n)
int *twosum(int *nums,int n,int target,int *returnSize)
{int i,*j;j=(int *)malloc(sizeof(int)*2);HashTable *ht;ht=(HashTable *)malloc(sizeof(HashTable));Init_HT(ht,n);for(i=0;i<n;i++)Insert_Item_HT(ht,i,nums[i]);for(i=0;i<n;i++){int k;k=Search_Item_HT(ht,target-nums[i]);if(k>=0 && k!=i){j[0]=i;j[1]=k;*returnSize=2;return j;}}*returnSize=0;return NULL;
}
//哈希表初始化
void Init_HT(HashTable *ht,int n)
{ht->len=n;ht->B=(Node **)malloc(sizeof(Node *)*n);int i;for(i=0;i<n;i++)ht->B[i]=NULL;
}
//释放哈希表
void Free_HT(HashTable *ht)
{int i;for(i=0;i<ht->len;i++){Node *cur,*pre;cur=(Node *)malloc(sizeof(Node));pre=(Node *)malloc(sizeof(Node));cur=ht->B[i];while(cur){pre=cur;cur=cur->next;free(pre);}}free(ht);
}
//向哈希表插入元素
void Insert_Item_HT(HashTable *ht,int index,int num)
{int i;i=num%ht->len;Node *p;p=(Node *)malloc(sizeof(Node));p->index=index;p->num=num;if(ht->B[i]){p->next=ht->B[i]->next;ht->B[i]->next=p;}else{ht->B[i]=p;p->next=NULL;}
}
//从哈希表中查找元素
int Search_Item_HT(HashTable *ht,int num)
{int i;i=num%ht->len;Node *p;p=(Node *)malloc(sizeof(Node));p=ht->B[i];while(p){if(p->num==num)return p->index;elsep=p->next;}return -1;
}