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39 lines
1.5 KiB
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39 lines
1.5 KiB
HTML
<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>
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<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>
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<p> </p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> s = "1 + 1"
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<strong>Output:</strong> 2
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> s = " 2-1 + 2 "
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<strong>Output:</strong> 3
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> s = "(1+(4+5+2)-3)+(6+8)"
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<strong>Output:</strong> 23
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</pre>
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<p> </p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>1 <= s.length <= 3 * 10<sup>5</sup></code></li>
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<li><code>s</code> consists of digits, <code>'+'</code>, <code>'-'</code>, <code>'('</code>, <code>')'</code>, and <code>' '</code>.</li>
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<li><code>s</code> represents a valid expression.</li>
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<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>
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<li><code>'-'</code> could be used as a unary operation (i.e., <code>"-1"</code> and <code>"-(2 + 3)"</code> is valid).</li>
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<li>There will be no two consecutive operators in the input.</li>
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<li>Every number and running calculation will fit in a signed 32-bit integer.</li>
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</ul>
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