codeforces 552 E. Vanya and Brackets 表达式求值

题目链接

讲道理距离上一次写这种求值的题已经不知道多久了。

括号肯定是左括号在乘号的右边， 右括号在左边。 否则没有意义。 题目说乘号只有15个， 所以我们枚举就好了。

#include <iostream>
#include <vector>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <complex>
#include <cmath>
#include <map>
#include <set>
#include <string>
#include <queue>
#include <stack>
#include <bitset>
using namespace std;
#define pb(x) push_back(x)
#define ll long long
#define mk(x, y) make_pair(x, y)
#define lson l, m, rt<<1
#define mem(a) memset(a, 0, sizeof(a))
#define rson m+1, r, rt<<1|1
#define mem1(a) memset(a, -1, sizeof(a))
#define mem2(a) memset(a, 0x3f, sizeof(a))
#define rep(i, n, a) for(int i = a; i<n; i++)
#define fi first
#define se second
typedef complex <double> cmx;
typedef pair<int, int> pll;
const double PI = acos(-1.0);
const double eps = 1e-8;
const int mod = 1e9+7;
const int inf = 1061109567;
const int dir[][2] = { {-1, 0}, {1, 0}, {0, -1}, {0, 1} };
stack <char> sign;
stack <ll> digit;
int a[20];
string str;
void cal() {
    char s = sign.top(); sign.pop();
    ll x = digit.top(); digit.pop();
    ll y = digit.top(); digit.pop();
    ll tmp;
    if(s == '+')
        tmp = x+y;
    else
        tmp = x*y;
    digit.push(tmp);
}
ll solve(){
    if(!digit.empty())
        digit.pop();
    for(int i = 0; i < str.size(); i ++) {
        if(isdigit(str[i])) {
            digit.push(str[i]-'0');
        } else if(str[i] == '(') {
            sign.push('(');
        } else if(str[i] == ')') {
            while(sign.top() != '(') {
                cal();
            }
            sign.pop();
        } else if(str[i] == '*') {
            sign.push('*');
        } else {
            while(!sign.empty() && sign.top() == '*')
                cal();
            sign.push('+');
        }
    }
    while(!sign.empty())
        cal();
    return digit.top();
}
int main()
{
    string s;
    cin>>s;
    int cnt = 0;
    a[cnt++] = -1;
    for(int i = 0; i < s.size(); i++)
        if(s[i] == '*')
            a[cnt++] = i;
    a[cnt++] = s.size();
    ll ans = 0;
    for(int i = 0; i < cnt; i ++) {
        for(int j = i+1; j < cnt; j ++) {
            str = s;
            str.insert(a[i]+1, 1, '(');
            str.insert(a[j]+1, 1, ')');
            ans = max(ans, solve());
        }
    }
    cout<<ans<<endl;
    return 0;
}