pat A1041 Be Unique

          pat A1041 Be Unique
subject ：
Being unique is so important to people on Mars that even their lottery is designed in a unique way. The rule of winning is simple: one bets on a number chosen from [1,10^4]. The first one who bets on a unique number wins. For example, if there are 7 people betting on { 5 31 5 88 67 88 17 }, then the second one who bets on 31 wins.

Input Specification:
Each input file contains one test case. Each case contains a line which begins with a positive integer N (≤10^5) and then followed by N bets. The numbers are separated by a space.

Output Specification:
For each test case, print the winning number in a line. If there is no winner, print None instead.

Sample Input 1:
7 5 31 5 88 67 88 17
Sample Output 1:
31

Sample Input 2:
5 888 666 666 888 888
Sample Output 2:
None

link ：pat A1041

#include <stdio.h>
int a[100010];
int hashTable[10010] = {
    };

int main(){
    
    int n;
    scanf("%d", &n);
    // Read in all numbers 
    for(int i = 0; i < n; i++){
    
        // Order hash ---> linear transformation 
        scanf("%d", &a[i]);
        // Count hash ---> Identity transformation 
        hashTable[a[i]]++;
    }
    // determine 
    int ans = -1;
    for(int i = 0; i < n; i++){
    
        if(hashTable[a[i]] == 1){
    
            ans = a[i];
            break;
        }
    }
    // Output 
    if(ans == -1){
    
        printf("None");
    }else{
    
        printf("%d", ans);
    }
    return 0;
}

summary ：

1. There are three useful data in this question :
      ① Order of input
      ② Specific values
      ③ The number of occurrences of each specific value .
   Now use the appropriate hash function and array (hashTable) Connect these three values .【 Understand hashes from a special perspective ： Connect useful data gracefully 】【 And three useful data need to be connected twice 】.
      For order , Just distinguish the order , There is no need for exact order , Therefore, the direct addressing method of integer hash is used -> linear transformation ,H( order ） = order -1,H(key1) Recorded in the array a Of the key values .
      For specific values , The direct addressing method of integer hash is used -> Identity transformation ,H(key2) Recorded in the array hashTable Of the key values .
2. hash The length of an array is always be equal to Max(H(Key)) + 10.