0150_evaluate_reverse_polish_notation.html

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    <title>LC 150: Evaluate Reverse Polish Notation - Algorithm Visualization</title>
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            <h1><span class="problem-number">#150</span> Evaluate Reverse Polish Notation</h1>
            <p>Evaluate the value of an arithmetic expression in Reverse Polish Notation (postfix notation). Valid operators are +, -, *, and /.</p>
            <div class="problem-meta">
                <span class="meta-tag">📚 Stack</span>
                <span class="meta-tag">🔢 Math</span>
                <span class="meta-tag">⏱️ O(n)</span>
                <span class="meta-tag">💾 O(n)</span>
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            <div class="file-ref">
                📄 Python: <code>python/0150_evaluate_reverse_polish_notation/0150_evaluate_reverse_polish_notation.py</code>
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            </div>

        <div class="explanation-panel">
            <h4>🧠 How It Works (Layman's Terms)</h4>
            <p>RPN (Reverse Polish Notation) puts operators AFTER operands. Here's how to evaluate it:</p>
            <ul>
                <li><strong>See a number?</strong> Push it onto the stack</li>
                <li><strong>See an operator?</strong> Pop two numbers, apply the operator, push the result</li>
                <li><strong>Important:</strong> The SECOND popped value is the LEFT operand (a op b, not b op a)</li>
                <li><strong>At the end:</strong> Stack has exactly one element - the answer!</li>
            </ul>
            <p><strong>Example:</strong> "2 1 +" means 2 + 1 = 3. "3 4 *" means 3 * 4 = 12.</p>
        </div>


        <div class="visualization-section">
            <h3>🎬 Step-by-Step Visualization</h3>
            
            <div class="controls">
                <select id="exampleSelect" style="padding: 8px; border-radius: 5px; border: 2px solid #ddd;">
                    <option value="0">["2","1","+","3","*"] = 9</option>
                    <option value="1">["4","13","5","/","+"] = 6</option>
                    <option value="2">["4","3","-"] = 1</option>
                </select>
                <button class="btn btn-primary" onclick="loadExample()">Load</button>
                <button class="btn btn-primary" onclick="step()">Step</button>
                <button class="btn btn-success" onclick="autoRun()">Auto Run</button>
                <button class="btn" style="background: #607d8b; color: white;" onclick="reset()">Reset</button>
            </div>

            <div class="status-message" id="statusMessage">
                Click Step to evaluate the expression token by token
            </div>

            <div style="margin-top: 20px;">
                <h4 style="margin-bottom: 10px;">📝 Tokens</h4>
                <div id="tokensContainer" style="display: flex; gap: 8px; flex-wrap: wrap; padding: 15px; background: #f5f5f5; border-radius: 12px;">
                </div>
            </div>

            <div style="display: flex; gap: 30px; flex-wrap: wrap; margin-top: 20px;">
                <div style="flex: 1; min-width: 200px;">
                    <h4 style="margin-bottom: 10px;">📚 Stack</h4>
                    <div id="stackContainer" style="display: flex; flex-direction: column-reverse; gap: 5px; padding: 20px; background: #e3f2fd; border-radius: 12px; min-height: 200px;">
                        <span style="color: #999; text-align: center;">Empty</span>
                    </div>
                </div>
                <div style="flex: 1; min-width: 250px;">
                    <h4 style="margin-bottom: 10px;">🧮 Current Operation</h4>
                    <div id="operationContainer" style="padding: 20px; background: #fff3e0; border-radius: 12px; min-height: 150px; text-align: center;">
                        <span style="color: #999;">-</span>
                    </div>
                    
                    <h4 style="margin-top: 20px; margin-bottom: 10px;">✅ Final Result</h4>
                    <div id="resultContainer" style="padding: 20px; background: #e8f5e9; border-radius: 12px; text-align: center; font-size: 2em; font-weight: bold; color: #4caf50;">
                        -
                    </div>
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        <div class="code-section">
            <h3>💻 Python Solution</h3>
            <div class="code-block">
                <pre>from typing import List, Optional

"""
LeetCode Evaluate Reverse Polish Notation

Problem from LeetCode: https://leetcode.com/problems/evaluate-reverse-polish-notation/

Description:
Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, and /. Each operand may be an integer or another expression.

Note that division between two integers should truncate toward zero.

It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.

Example 1:
Input: tokens = ["2","1","+","3","*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:
Input: tokens = ["4","13","5","/","+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:
Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
Output: 22
Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22
"""

class Solution:
    def eval_r_p_n(self, tokens: List[str]) -&gt; int:
        """
        Evaluate the value of an arithmetic expression in Reverse Polish Notation.
        
        Args:
            tokens: List of tokens in Reverse Polish Notation
            
        Returns:
            int: Result of the evaluated expression
        """
        stack = []
        for token in tokens:
            if self.is_operator(token):
                b = stack.pop()
                a = stack.pop()
                result = self.apply_operator(token, a, b)
                stack.append(result)
            else:
                stack.append(int(token))
        return stack[0]

    def is_operator(self, token: str) -&gt; bool:
        """
        Check if a token is an operator.
        
        Args:
            token: Token to check
            
        Returns:
            bool: True if token is an operator, False otherwise
        """
        return token in ['+', '-', '*', '/']

    def apply_operator(self, operator: str, a: int, b: int) -&gt; int:
        """
        Apply an arithmetic operator to two operands.
        
        Args:
            operator: Arithmetic operator (+, -, *, /)
            a: First operand
            b: Second operand
            
        Returns:
            int: Result of the operation a operator b
        """
        if operator == '+':
            return a + b
        elif operator == '-':
            return a - b
        elif operator == '*':
            return a * b
        elif operator == '/':
            return int(a / b)
        else:
            raise ValueError('Invalid operator')


if __name__ == '__main__':
    # Example usage based on LeetCode sample
    solution = Solution()
    
    # Example 1
    tokens1 = ["2", "1", "+", "3", "*"]
    result1 = solution.eval_r_p_n(tokens1)
    print(f"Example 1: {tokens1} -&gt; {result1}")  # Expected output: 9
    
    # Example 2
    tokens2 = ["4", "13", "5", "/", "+"]
    result2 = solution.eval_r_p_n(tokens2)
    print(f"Example 2: {tokens2} -&gt; {result2}")  # Expected output: 6
    
    # Example 3
    tokens3 = ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
    result3 = solution.eval_r_p_n(tokens3)
    print(f"Example 3: Result = {result3}")  # Expected output: 22
    
    # Additional example with negative numbers
    tokens4 = ["4", "3", "-"]
    result4 = solution.eval_r_p_n(tokens4)
    print(f"Example 4: {tokens4} -&gt; {result4}")  # Expected output: 1
</pre>
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    <script>
        const examples = [
            ["2", "1", "+", "3", "*"],
            ["4", "13", "5", "/", "+"],
            ["4", "3", "-"]
        ];

        let tokens = [...examples[0]];
        let stack = [];
        let currentIdx = 0;
        let done = false;
        let isRunning = false;

        function loadExample() {
            const idx = parseInt(document.getElementById('exampleSelect').value);
            tokens = [...examples[idx]];
            reset();
        }

        function isOperator(token) {
            return ['+', '-', '*', '/'].includes(token);
        }

        function renderTokens() {
            const container = document.getElementById('tokensContainer');
            container.innerHTML = tokens.map((t, i) => {
                let bgColor = '#fff';
                let borderColor = '#ddd';
                let textColor = '#333';

                if (i < currentIdx) {
                    bgColor = '#e0e0e0';
                    textColor = '#999';
                } else if (i === currentIdx) {
                    bgColor = isOperator(t) ? '#ff9800' : '#667eea';
                    borderColor = isOperator(t) ? '#f57c00' : '#5a6fd6';
                    textColor = 'white';
                }

                return `
                    <div style="padding: 12px 18px; background: ${bgColor}; border: 2px solid ${borderColor}; 
                                border-radius: 8px; font-weight: bold; font-size: 1.2em; color: ${textColor};
                                min-width: 40px; text-align: center;">
                        ${t}
                        ${i === currentIdx ? '<div style="font-size: 0.6em; margin-top: 2px;">▲</div>' : ''}
                    </div>
                `;
            }).join('');
        }

        function renderStack() {
            const container = document.getElementById('stackContainer');
            if (stack.length === 0) {
                container.innerHTML = '<span style="color: #999; text-align: center;">Empty</span>';
                return;
            }
            container.innerHTML = stack.map((val, i) => {
                const isTop = i === stack.length - 1;
                return `
                    <div style="padding: 12px 20px; background: ${isTop ? '#667eea' : '#fff'}; 
                                border: 2px solid ${isTop ? '#5a6fd6' : '#90caf9'}; border-radius: 8px;
                                font-weight: bold; font-size: 1.2em; color: ${isTop ? 'white' : '#333'};
                                text-align: center;">
                        ${val}
                        ${isTop ? '<span style="margin-left: 10px; font-size: 0.8em;">← TOP</span>' : ''}
                    </div>
                `;
            }).reverse().join('');
        }

        function renderOperation(op, a, b, result) {
            const container = document.getElementById('operationContainer');
            if (!op) {
                container.innerHTML = '<span style="color: #999;">-</span>';
                return;
            }

            const opSymbol = op === '*' ? '×' : op === '/' ? '÷' : op;
            container.innerHTML = `
                <div style="font-size: 1.5em; color: #666;">
                    <span style="color: #667eea; font-weight: bold;">${a}</span>
                    <span style="color: #ff9800; margin: 0 10px;">${opSymbol}</span>
                    <span style="color: #667eea; font-weight: bold;">${b}</span>
                </div>
                <div style="font-size: 2em; margin: 10px 0;">=</div>
                <div style="font-size: 2em; color: #4caf50; font-weight: bold;">${result}

        </div>
            `;
        }

        function step() {
            if (done) {
                document.getElementById('statusMessage').textContent = 
                    `Done! Final answer: ${stack[0]}`;
                return;
            }

            if (currentIdx >= tokens.length) {
                done = true;
                document.getElementById('resultContainer').textContent = stack[0];
                document.getElementById('statusMessage').textContent = 
                    `Complete! The expression evaluates to ${stack[0]}`;
                return;
            }

            const token = tokens[currentIdx];

            if (isOperator(token)) {
                const b = stack.pop();
                const a = stack.pop();
                let result;

                switch (token) {
                    case '+': result = a + b; break;
                    case '-': result = a - b; break;
                    case '*': result = a * b; break;
                    case '/': result = Math.trunc(a / b); break;
                }

                stack.push(result);
                renderOperation(token, a, b, result);
                document.getElementById('statusMessage').textContent = 
                    `Operator "${token}": Pop ${b} and ${a}, compute ${a} ${token} ${b} = ${result}, push ${result}`;
            } else {
                const num = parseInt(token);
                stack.push(num);
                renderOperation(null);
                document.getElementById('statusMessage').textContent = 
                    `Number "${token}": Push ${num} onto stack`;
            }

            currentIdx++;
            renderTokens();
            renderStack();

            if (currentIdx >= tokens.length) {
                document.getElementById('resultContainer').textContent = stack[0];
            }
        }

        function autoRun() {
            if (isRunning) return;
            isRunning = true;

            const interval = setInterval(() => {
                if (done || currentIdx >= tokens.length) {
                    clearInterval(interval);
                    isRunning = false;
                    if (stack.length > 0) {
                        document.getElementById('resultContainer').textContent = stack[0];
                        document.getElementById('statusMessage').textContent = 
                            `Complete! The expression evaluates to ${stack[0]}`;
                    }
                    done = true;
                    return;
                }
                step();
            }, 1000);
        }

        function reset() {
            stack = [];
            currentIdx = 0;
            done = false;
            isRunning = false;

            document.getElementById('statusMessage').textContent = 
                'Click Step to evaluate the expression token by token';
            document.getElementById('resultContainer').textContent = '-';
            
            renderTokens();
            renderStack();
            renderOperation(null);
        }

        reset();
    </script>
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