题解 | 表达式求值

class Solution {
public:
    int solve(string s) {
        stack<int> val;
        stack<char> ops;
        for(int i=0; i<=s.length();){
            if(s[i]>='0'&&s[i]<='9')
                val.push(toInt(s, i));
            else if(ops.empty()||ops.top()=='('||s[i]=='(')
                ops.push(s[i++]);
            else if(s[i]=='*'){
                int t1=val.top(),t2=toInt(s, ++i);
                val.pop();
                val.push(t1*t2);
            }
            else{
                int t2=val.top();
                val.pop();
                int t1=val.top();
                val.pop();
                if(ops.top()=='+')
                    val.push(t1+t2);
                else if(ops.top()=='-')
                    val.push(t1-t2);
                else if(ops.top()=='*')
                    val.push(t1*t2);
                ops.pop();
                if(s[i]==')'){
                    ops.pop();
                    i++;
                }
                else if(i==s.length())
                    break;
            }
        }
        return val.top();
    }
    int toInt(string s,int &i){
        int tmp=0;
        while(s[i]<='9'&&s[i]>='0')
            tmp=tmp*10+s[i++]-'0';
        return tmp;
    }
};