<p>Given a string <code>s</code> representing a valid expression, implement a basic calculator to evaluate it, and return <em>the result of the evaluation</em>.</p>

<p><strong>Note:</strong> You are <strong>not</strong> allowed to use any built-in function which evaluates strings as mathematical expressions, such as <code>eval()</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;1 + 1&quot;
<strong>Output:</strong> 2
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot; 2-1 + 2 &quot;
<strong>Output:</strong> 3
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;(1+(4+5+2)-3)+(6+8)&quot;
<strong>Output:</strong> 23
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 3 * 10<sup>5</sup></code></li>
	<li><code>s</code> consists of digits, <code>&#39;+&#39;</code>, <code>&#39;-&#39;</code>, <code>&#39;(&#39;</code>, <code>&#39;)&#39;</code>, and <code>&#39; &#39;</code>.</li>
	<li><code>s</code> represents a valid expression.</li>
	<li><code>&#39;+&#39;</code> is <strong>not</strong> used as a unary operation (i.e., <code>&quot;+1&quot;</code> and <code>&quot;+(2 + 3)&quot;</code> is invalid).</li>
	<li><code>&#39;-&#39;</code> could be used as a unary operation (i.e., <code>&quot;-1&quot;</code> and <code>&quot;-(2 + 3)&quot;</code> is valid).</li>
	<li>There will be no two consecutive operators in the input.</li>
	<li>Every number and running calculation will fit in a signed 32-bit integer.</li>
</ul>

class Solution {
public:
    int precedence(char ch) {
        if(ch == '+' || ch == '-') {
            return 1;
        }
        if(ch == 'X') {
            return 2;
        }
        return 0; // 0 for (
    }

    bool isHigherPrecedence(char curr, char prev) {
        if(curr == '+' || curr == '-') {
            return precedence(curr) > precedence(prev);
        }
        return precedence(curr) >= precedence(prev);
    }

    bool isDigit(char ch) {
        return ch >= '0' && ch <= '9';
    }

    string converToPostfix(string s) {
        string expr = "";
        stack<char> ops;
        int n = s.size();
        bool wasLastOperator = true;
        for(int i=0;i<n;i++) {
            if(s[i] == ' ') {
                continue;
            }
            if(isDigit(s[i])) {
                expr += s[i];
                if(i == n-1 || !isDigit(s[i+1])) {
                    expr += "_";
                }
                wasLastOperator = false;
                continue;
            }
            if(s[i] == '(') {
                ops.push('(');
            }
            else if(s[i] == ')') {
                while(ops.top() != '(') {
                    expr += ops.top();
                    ops.pop();
                }
                ops.pop();
            } else {
                if(s[i] == '-' && wasLastOperator) {
                    s[i] = 'X';
                }
                while(!ops.empty() && !isHigherPrecedence(s[i], ops.top())) {
                    expr += ops.top();
                    ops.pop();
                }
                ops.push(s[i]);
                wasLastOperator = true;
            }
        }
        while(!ops.empty()) {
            expr += ops.top();
            ops.pop();
        }
        return expr;
    }

    int evalPostfixExpr(string s) {
        stack<int> operands;
        int n = s.size(), op1, op2;
        string curr = "";
        for(int i=0;i<n;i++) {
            if(isDigit(s[i])) {
                curr += s[i];
            }else if(s[i] == '_') {
                operands.push(stoi(curr));
                curr = "";
            } else if(s[i] == '+') {
                op2 = operands.top();
                operands.pop();
                op1 = operands.top();
                operands.pop();
                operands.push(op1 + op2);
            } else if(s[i] == '-') {
                op2 = operands.top();
                operands.pop();
                op1 = operands.top();
                operands.pop();
                operands.push(op1 - op2);
            } else if (s[i] == 'X') {
                op1 = operands.top();
                operands.pop();
                operands.push(op1*-1);
            }
        }
        return operands.top();
    }

    int calculate(string s) {
        string expr = converToPostfix(s);
        // cout<<"Postfix - "<<expr<<endl;
        return evalPostfixExpr(expr);
    }
};
14056