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解题思路：字符串处理，我这里完全是手动判断，注意2.也是合法的浮点数，后面有个点，我分了两个小数和一个小数，最后还要加没有小数部分只有小数点的情况。

#include<cstdio>
#include<cstring>
double ans;
int check(char s[]){
    int len=strlen(s);
    double tmp=0;   
    int tag=1;
    for(int i=0;i<len;i++){
        if(s[i]=='-'){
            i++;
            tag=2;
        } 
        if((s[i]>='0'&&s[i]<='9')||s[i]=='.'){
            if(s[i]=='.'){
                if(s[i+1]>='0'&&s[i+1]<='9'&&s[i+2]>='0'&&s[i+2]<='9'&&i==len-3){
                     ans = tmp+(s[i+1]-'0')*0.1 +(s[i+2]-'0')*0.01;
                     if(tag==2) ans=-ans;
                     return 1;
                }else if(s[i+1]>='0'&&s[i+1]<='9'&&i==len-2){
                     ans = tmp+(s[i+1]-'0')*0.1;
                     if(tag==2) ans=-ans;
                     return 1;
                }else if(i==len-1){
                    ans=tmp;
                    if(tag==2) ans=-ans;
                     return 1;
                }else return 0;
            }else{
                tmp = tmp*10 + s[i]-'0';
            }
        }else return 0;
    }
    ans = tmp;
    if(tag==2) ans = -ans;
    return 1;
}
int main(){
    int N;
    scanf("%d",&N);
    char str[100];
    int count = 0;
    double sum=0;
    for(int i=0;i<N;i++){
        ans=0;
        scanf("%s",str);
        if(check(str)==0||ans<-1000||ans>1000)
            printf("ERROR: %s is not a legal number\n",str);
        else{
            sum += ans;
            count++;
        }
    }
    if(count==0)
        printf("The average of 0 numbers is Undefined\n");
    else if(count==1)
        printf("The average of 1 number is %.2f\n",sum);
    else
        printf("The average of %d numbers is %.2f\n",count,sum/count);
    return 0;
}