思路与算法

读到行尾/EOF，流式维护最小值、最大值、总和和个数。平均值按总和的绝对值乘1000再除个数，保留商和余数，按最近值、恰好一半时末位取偶的标准浮点输出舍入规则处理，最后补足3位小数及符号。

正确性说明

每次读数更新区间极值与累计和，归纳后即为所有输入的统计量。商是平均值绝对值放大1000后的整数部分，余数与除数的一半比较决定最接近的千分位；正负对称处理保证负数准确。大整数避免输入只声明不超过1000却未声明负数下界时的截断。

复杂度

O(qD)时间、O(D)额外空间，q≤100，D为输入与总和位数；含极值、累计值和十进制格式化初始化。

易错点

输入没有预先给个数，不能把第一个数当n。平均值保留3位，最小最大值仍输出整数；不在每次读入时提前舍入。

参考代码（C++11）

#include <bits/stdc++.h>
using namespace std;
struct cpp_int {
    string s;
    bool neg;
    cpp_int(long long v = 0) : s(to_string(v)), neg(v < 0) {
        if (neg)
            s.erase(s.begin());
    }
    void norm() {
        auto p = s.find_first_not_of('0');
        s = p == string::npos ? "0" : s.substr(p);
        if (s == "0")
            neg = false;
    }
    static int cmpabs(const cpp_int &a, const cpp_int &b) {
        if (a.s.size() != b.s.size())
            return a.s.size() < b.s.size() ? -1 : 1;
        return a.s == b.s ? 0 : a.s < b.s ? -1 : 1;
    }
    friend bool operator<(const cpp_int &a, const cpp_int &b) {
        if (a.neg != b.neg)
            return a.neg;
        int c = cmpabs(a, b);
        return a.neg ? c > 0 : c < 0;
    }
    friend bool operator==(const cpp_int &a, const cpp_int &b) {
        return a.neg == b.neg && a.s == b.s;
    }
    friend bool operator!=(const cpp_int &a, const cpp_int &b) {
        return !(a == b);
    }
    friend bool operator>(const cpp_int &a, const cpp_int &b) {
        return b < a;
    }
    friend bool operator<=(const cpp_int &a, const cpp_int &b) {
        return !(b < a);
    }
    friend bool operator>=(const cpp_int &a, const cpp_int &b) {
        return !(a < b);
    }
    cpp_int operator-() const {
        cpp_int r = *this;
        if (r.s != "0")
            r.neg = !r.neg;
        return r;
    }
    friend cpp_int operator+(const cpp_int &a, const cpp_int &b) {
        cpp_int r;
        r.s.clear();
        int carry = 0;
        if (a.neg == b.neg) {
            int i = (int)a.s.size() - 1, j = (int)b.s.size() - 1;
            while (i >= 0 || j >= 0 || carry) {
                int v = carry + (i >= 0 ? a.s[i--] - '0' : 0) + (j >= 0 ? b.s[j--] - '0' : 0);
                r.s += char('0' + v % 10);
                carry = v / 10;
            }
            r.neg = a.neg;
        } else {
            const cpp_int *x = &a, *y = &b;
            if (cmpabs(a, b) < 0)
                swap(x, y);
            int i = (int)x->s.size() - 1, j = (int)y->s.size() - 1;
            while (i >= 0) {
                int v = x->s[i--] - '0' - carry - (j >= 0 ? y->s[j--] - '0' : 0);
                carry = v < 0;
                if (carry)
                    v += 10;
                r.s += char('0' + v);
            }
            r.neg = x->neg;
        }
        reverse(r.s.begin(), r.s.end());
        r.norm();
        return r;
    }
    friend cpp_int operator-(const cpp_int &a, const cpp_int &b) {
        return a + (-b);
    }
    friend cpp_int operator*(const cpp_int &a, int b) {
        cpp_int r;
        r.s.clear();
        long long carry = 0;
        long long z = b;
        if (z < 0)
            z = -z;
        for (int i = (int)a.s.size() - 1; i >= 0; i--) {
            carry += (a.s[i] - '0') * z;
            r.s += char('0' + carry % 10);
            carry /= 10;
        }
        while (carry) {
            r.s += char('0' + carry % 10);
            carry /= 10;
        }
        reverse(r.s.begin(), r.s.end());
        r.neg = a.neg ^ (b < 0);
        r.norm();
        return r;
    }
    friend cpp_int operator/(const cpp_int &a, int b) {
        cpp_int r;
        r.s.clear();
        long long rem = 0, z = b;
        if (z < 0)
            z = -z;
        for (char c : a.s) {
            rem = rem * 10 + c - '0';
            r.s += char('0' + rem / z);
            rem %= z;
        }
        r.neg = a.neg ^ (b < 0);
        r.norm();
        return r;
    }
    cpp_int &operator+=(const cpp_int &b) {
        *this = *this + b;
        return *this;
    }
    cpp_int &operator-=(const cpp_int &b) {
        *this = *this - b;
        return *this;
    }
    cpp_int &operator*=(int b) {
        *this = *this * b;
        return *this;
    }
    friend istream &operator>>(istream &in, cpp_int &a) {
        string v;
        if (in >> v) {
            a.neg = v[0] == '-';
            if (v[0] == '-' || v[0] == '+')
                v.erase(v.begin());
            a.s = v;
            a.norm();
        }
        return in;
    }
    friend ostream &operator<<(ostream &out, const cpp_int &a) {
        if (a.neg)
            out << '-';
        return out << a.s;
    }
};
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cpp_int x, sum, mn, mx;
    int n = 0;
    while (cin >> x) {
        if (!n)
            mn = mx = x;
        mn = min(mn, x);
        mx = max(mx, x);
        sum += x;
        n++;
    }
    bool negative = sum < 0;
    if (negative)
        sum = -sum;
    cpp_int scaled = sum * 1000, q = scaled / n;
    int rem = 0;
    for (char c : scaled.s)
        rem = (rem * 10 + c - '0') % n;
    if (rem * 2 > n || (rem * 2 == n && (q.s.back() - '0') % 2))
        q += 1;
    string s = q.s;
    while (s.size() < 4)
        s = '0' + s;
    cout << mn << " " << mx << " " << (negative ? "-" : "") << s.substr(0, s.size() - 3) << "."
         << s.substr(s.size() - 3) << "\n";
}