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Problem solving Techniques

use static unordered_set<...> varName = { ... } and along with find() to find if that element is present or not

static unordered_set<string> ops = {"+", "-", "*", "/"};

class Solution {
public:
    int RPN_Postfix_to_Int(vector<string>& tokens) {
        stack<int> stack;
        for (auto t : tokens) {
            if (ops.find(t) == ops.end()) {
                stack.push(stoi(t));
            }
            else {
                int b = stack.top(); stack.pop();
                int a = stack.top(); stack.pop();
                if (t == "+") stack.push(a + b);
                else if (t == "-") stack.push(a - b);
                else if (t == "*") stack.push(a * b);
                else stack.push(a / b);
            }
        }
        return stack.top();
    }
};

use simple vector or array to store conditions of if​

vector<int> v1(3);
vector<int> v2(3);

v1[0] = ...;
v1[1] = ...;
v1[3] = ...;

/*
and so on ...
 */

switch case instead of if​

int RPN_Postfix_to_Int(vector<string>& tokens) {
    stack<int> stn;
    for(auto s:tokens) {
        if(s.size()>1 || isdigit(s[0])) stn.push(stoi(s));
        else {
            auto x2=stn.top(); stn.pop();
            auto x1=stn.top(); stn.pop();
            switch(s[0]) {
                case '+': x1+=x2; break;
                case '-': x1-=x2; break;
                case '*': x1*=x2; break;
                case '/': x1/=x2; break;
            }
            stn.push(x1);
        }
    }
    return stn.top();
}

use for each loop wherever possible​

int RPN_Postfix_to_Int(vector<string>& tokens) {
    stack<int> stn;
    for(auto s:tokens) {
        if(s.size()>1 || isdigit(s[0])) stn.push(stoi(s));
        else {
            auto x2=stn.top(); stn.pop();
            auto x1=stn.top(); stn.pop();
            switch(s[0]) {
                case '+': x1+=x2; break;
                case '-': x1-=x2; break;
                case '*': x1*=x2; break;
                case '/': x1/=x2; break;
            }
            stn.push(x1);
        }
    }
    return stn.top();
}

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### Some Tips

- when given sequences, always take conditions for `return` in :
  - empty .. no element
  - only 1 element
  - always consider **dry coding** your algo with atleast first 2 elements
  - the condition provided