https://www.acwing.com/problem/content/3305/
input:
(2+2)*(1+1)
output:
8
#include<unordered_map>
const int N = 1e5 + 50;
stack<int>num;
stack<char>op;
char s[N];
void eval(){
int b = num.top();
num.pop();
int a = num.top();
num.pop();
char c = op.top();
op.pop();
int x = 0;
if(c == '+')x=a+b;
else if(c == '-')x=a-b;
else if(c == '*')x=a*b;
else if(c == '/')x=a/b;
num.push(x);
}
void work(){
unordered_map<char,int>pr{{'+',1},{'-',1},{'*',2},{'/',2}};
scanf("%s",s);
int lena = strlen(s);
for(int i=0;i<lena;i++){
char c = s[i];
if(isdigit(c)){
int x=0,j=i;
while(j<lena&&isdigit(s[j])){
x=x*10+(s[j]-'0');
j++;
}
i=j-1;
num.push(x);
}
else if(c == '(')op.push(c);
else if(c == ')'){
while(op.top() != '(')eval();
op.pop();
}
else {
while(op.size() && pr[op.top()] >= pr[c])eval();
op.push(c);
}
}
while(op.size())eval();
printf("%d
",num.top());
}