<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> </p> <p><strong class="example">Example 1:</strong></p> <pre> <strong>Input:</strong> s = "1 + 1" <strong>Output:</strong> 2 </pre> <p><strong class="example">Example 2:</strong></p> <pre> <strong>Input:</strong> s = " 2-1 + 2 " <strong>Output:</strong> 3 </pre> <p><strong class="example">Example 3:</strong></p> <pre> <strong>Input:</strong> s = "(1+(4+5+2)-3)+(6+8)" <strong>Output:</strong> 23 </pre> <p> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>1 <= s.length <= 3 * 10<sup>5</sup></code></li> <li><code>s</code> consists of digits, <code>'+'</code>, <code>'-'</code>, <code>'('</code>, <code>')'</code>, and <code>' '</code>.</li> <li><code>s</code> represents a valid expression.</li> <li><code>'+'</code> is <strong>not</strong> used as a unary operation (i.e., <code>"+1"</code> and <code>"+(2 + 3)"</code> is invalid).</li> <li><code>'-'</code> could be used as a unary operation (i.e., <code>"-1"</code> and <code>"-(2 + 3)"</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>