algorithm-neet-code-top-150-python-visual/visual/0141_linked_list_cycle.html at main · kimtth/algorithm-neet-code-top-150-python-visual · GitHub

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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>LC 141: Linked List Cycle - Algorithm Visualization</title>
    <link rel="stylesheet" href="styles.css">
    <script src="https://d3js.org/d3.v7.min.js"></script>
</head>
<body>
    <div class="container">
        <div class="problem-info">
            <h1><span class="problem-number">#141</span> Linked List Cycle</h1>
            <p>Given the head of a linked list, determine if there is a cycle in it. A cycle exists if some node can be reached again by following the next pointers.</p>
            <div class="problem-meta">
                <span class="meta-tag">🔗 Linked List</span>
                <span class="meta-tag">🐢🐇 Floyd's Algorithm</span>
                <span class="meta-tag">⏱️ O(n)</span>
                <span class="meta-tag">💾 O(1)</span>
            </div>
            <div class="file-ref">
                📄 Python: <code>python/0141_linked_list_cycle/0141_linked_list_cycle.py</code>
            </div>
            </div>

        <div class="explanation-panel">
            <h4>🧠 How It Works (Layman's Terms)</h4>
            <p>Imagine two runners on a circular track - one slow (🐢 tortoise) and one fast (🐇 hare):</p>
            <ul>
                <li><strong>Slow pointer:</strong> Moves 1 step at a time</li>
                <li><strong>Fast pointer:</strong> Moves 2 steps at a time</li>
                <li><strong>If there's a cycle:</strong> Fast will eventually catch up and meet slow</li>
                <li><strong>If no cycle:</strong> Fast will reach the end (null)</li>
                <li><strong>Why it works:</strong> In a cycle, fast gains 1 step per iteration, so they must meet!</li>
            </ul>
        </div>


        <div class="visualization-section">
            <h3>🎬 Step-by-Step Visualization</h3>

            <div class="controls">
                <button class="btn btn-primary" id="stepBtn" onclick="step()">Step</button>
                <button class="btn btn-success" id="autoBtn" onclick="toggleAuto()">Auto Run</button>
                <button class="btn btn-warning" onclick="reset()">Reset</button>
            </div>

            <div class="info-box secondary" style="margin-bottom: 20px;">
                🔗 List: <strong>[3 → 2 → 0 → -4] → (back to node 2)</strong> — Has a cycle!
            </div>

            <div class="status-message" id="statusMessage">
                Click "Step" or "Auto Run" to start visualization
            </div>

            <div id="linkedListContainer" style="width: 100%; height: 350px; margin-top: 20px;"></div>

            <div class="variable-section" style="margin: 20px 0; display: flex; gap: 30px; justify-content: center;">
                <div class="variable" style="background: #e3f2fd;">
                    <span class="var-name">🐢 slow</span>
                    <span class="var-value" id="slowValue">3</span>
                    <span class="var-desc">moves 1 step</span>
                </div>
                <div class="variable" style="background: #fce4ec;">
                    <span class="var-name">🐇 fast</span>
                    <span class="var-value" id="fastValue">2</span>
                    <span class="var-desc">moves 2 steps</span>
                </div>
                <div class="variable">
                    <span class="var-name">Iteration</span>
                    <span class="var-value" id="iterValue">0</span>
                </div>
            </div>
        </div>

        <div class="code-section">
            <h3>💻 Python Solution (Floyd's Tortoise and Hare)</h3>
            <div class="code-block">
                <pre><span class="keyword">def</span> <span class="function">has_cycle</span>(self, head: <span class="class-name">Optional</span>[<span class="class-name">ListNode</span>]) -> <span class="class-name">bool</span>:
    <span class="keyword">if</span> <span class="keyword">not</span> head:
        <span class="keyword">return</span> <span class="keyword">False</span>

    slow = head
    fast = head.next

    <span class="keyword">while</span> slow <span class="keyword">and</span> fast:
        <span class="keyword">if</span> <span class="keyword">not</span> fast <span class="keyword">or</span> <span class="keyword">not</span> fast.next:
            <span class="keyword">return</span> <span class="keyword">False</span>
        <span class="keyword">if</span> fast == slow:
            <span class="keyword">return</span> <span class="keyword">True</span>
        slow = slow.next
        fast = fast.next.next

    <span class="keyword">return</span> <span class="keyword">False</span></pre>
            </div>
        </div>
    </div>

    <script>
        // Linked list: 3 -> 2 -> 0 -> -4 -> (back to 2)
        const nodes = [
            { val: 3, id: 0 },
            { val: 2, id: 1 },
            { val: 0, id: 2 },
            { val: -4, id: 3 }
        ];
        const cycleStart = 1; // Node index where cycle starts (node with value 2)

        let slowIdx = 0;
        let fastIdx = 1;
        let iteration = 0;
        let autoInterval = null;
        let finished = false;

        function getNextIdx(idx) {
            if (idx === nodes.length - 1) {
                return cycleStart; // Cycle back
            }
            return idx + 1;
        }

        function init() {
            renderLinkedList();
            updateVariables();
        }

        function renderLinkedList() {
            const container = document.getElementById('linkedListContainer');
            const width = container.offsetWidth;
            const height = container.offsetHeight;

            d3.select('#linkedListContainer').selectAll('*').remove();

            const svg = d3.select('#linkedListContainer')
                .append('svg')
                .attr('width', width)
                .attr('height', height);

            // Position nodes in a shape that shows the cycle
            const centerX = width / 2;
            const centerY = height / 2;
            const nodeRadius = 35;

            // Create positions: 3 → 2 → 0 → -4 with -4 looping back to 2
            const positions = [
                { x: centerX - 180, y: centerY - 50 },  // Node 0 (val: 3)
                { x: centerX - 60, y: centerY - 50 },   // Node 1 (val: 2) - cycle start
                { x: centerX + 60, y: centerY + 30 },   // Node 2 (val: 0)
                { x: centerX - 60, y: centerY + 110 }   // Node 3 (val: -4)
            ];

            // Draw arrows
            const arrowHead = svg.append('defs').append('marker')
                .attr('id', 'arrowhead')
                .attr('viewBox', '0 0 10 10')
                .attr('refX', 8)
                .attr('refY', 5)
                .attr('markerWidth', 6)
                .attr('markerHeight', 6)
                .attr('orient', 'auto')
                .append('path')
                .attr('d', 'M 0 0 L 10 5 L 0 10 Z')
                .attr('fill', '#666');

            // Regular arrows
            for (let i = 0; i < nodes.length - 1; i++) {
                const start = positions[i];
                const end = positions[i + 1];
                const angle = Math.atan2(end.y - start.y, end.x - start.x);

                svg.append('line')
                    .attr('x1', start.x + nodeRadius * Math.cos(angle))
                    .attr('y1', start.y + nodeRadius * Math.sin(angle))
                    .attr('x2', end.x - nodeRadius * Math.cos(angle))
                    .attr('y2', end.y - nodeRadius * Math.sin(angle))
                    .attr('stroke', '#666')
                    .attr('stroke-width', 2)
                    .attr('marker-end', 'url(#arrowhead)');
            }

            // Cycle arrow (from -4 back to 2)
            const lastPos = positions[3];
            const cyclePos = positions[1];

            // Draw curved arrow for cycle
            svg.append('path')
                .attr('d', `M ${lastPos.x - 20} ${lastPos.y - 20}
                           Q ${lastPos.x - 100} ${lastPos.y - 80}
                           ${cyclePos.x - 20} ${cyclePos.y + 20}`)
                .attr('fill', 'none')
                .attr('stroke', '#e91e63')
                .attr('stroke-width', 2)
                .attr('stroke-dasharray', '5,5')
                .attr('marker-end', 'url(#arrowhead)');

            // Cycle label
            svg.append('text')
                .attr('x', lastPos.x - 100)
                .attr('y', lastPos.y - 60)
                .attr('fill', '#e91e63')
                .attr('font-size', '12px')
                .text('cycle!');

            // Draw nodes
            nodes.forEach((node, i) => {
                const pos = positions[i];
                const g = svg.append('g')
                    .attr('transform', `translate(${pos.x}, ${pos.y})`);

                // Node circle
                let fillColor = '#fff';
                let strokeColor = '#667eea';
                let strokeWidth = 3;

                if (slowIdx === i && fastIdx === i) {
                    fillColor = '#ab47bc'; // Purple when both are here
                    strokeWidth = 5;
                } else if (slowIdx === i) {
                    fillColor = '#2196f3'; // Blue for slow
                    strokeWidth = 5;
                } else if (fastIdx === i) {
                    fillColor = '#e91e63'; // Pink for fast
                    strokeWidth = 5;
                }

                if (i === cycleStart) {
                    strokeColor = '#e91e63';
                }

                g.append('circle')
                    .attr('r', nodeRadius)
                    .attr('fill', fillColor)
                    .attr('stroke', strokeColor)
                    .attr('stroke-width', strokeWidth);

                // Node value
                g.append('text')
                    .attr('text-anchor', 'middle')
                    .attr('dy', 5)
                    .attr('font-size', '18px')
                    .attr('font-weight', 'bold')
                    .attr('fill', (slowIdx === i || fastIdx === i) ? '#fff' : '#333')
                    .text(node.val);

                // Node index
                g.append('text')
                    .attr('text-anchor', 'middle')
                    .attr('dy', -45)
                    .attr('font-size', '12px')
                    .attr('fill', '#666')
                    .text(`index: ${i}`);

                // Pointer labels
                if (slowIdx === i && fastIdx === i) {
                    g.append('text')
                        .attr('text-anchor', 'middle')
                        .attr('dy', 55)
                        .attr('font-size', '14px')
                        .attr('fill', '#ab47bc')
                        .text('🐢🐇 MEET!');
                } else {
                    if (slowIdx === i) {
                        g.append('text')
                            .attr('text-anchor', 'middle')
                            .attr('dy', 55)
                            .attr('font-size', '14px')
                            .attr('fill', '#2196f3')
                            .text('🐢 slow');
                    }
                    if (fastIdx === i) {
                        g.append('text')
                            .attr('text-anchor', 'middle')
                            .attr('dy', fastIdx === slowIdx ? 75 : 55)
                            .attr('font-size', '14px')
                            .attr('fill', '#e91e63')
                            .text('🐇 fast');
                    }
                }
            });

            // Legend
            svg.append('circle').attr('cx', 30).attr('cy', 20).attr('r', 8).attr('fill', '#2196f3');
            svg.append('text').attr('x', 45).attr('y', 24).text('slow (🐢)').attr('font-size', '12px');
            svg.append('circle').attr('cx', 30).attr('cy', 45).attr('r', 8).attr('fill', '#e91e63');
            svg.append('text').attr('x', 45).attr('y', 49).text('fast (🐇)').attr('font-size', '12px');
        }

        function updateVariables() {
            document.getElementById('slowValue').textContent = nodes[slowIdx].val;
            document.getElementById('fastValue').textContent = nodes[fastIdx].val;
            document.getElementById('iterValue').textContent = iteration;
        }

        function step() {
            if (finished) return;

            iteration++;

            // Check if they meet
            if (slowIdx === fastIdx) {
                finished = true;
                document.getElementById('statusMessage').className = 'status-message success';
                document.getElementById('statusMessage').textContent =
                    `✅ CYCLE DETECTED! Slow and fast pointers meet at node ${nodes[slowIdx].val}`;
                document.getElementById('stepBtn').disabled = true;
                stopAuto();
                renderLinkedList();
                updateVariables();
                return;
            }

            // Move slow by 1
            const oldSlow = slowIdx;
            slowIdx = getNextIdx(slowIdx);

            // Move fast by 2
            const oldFast = fastIdx;
            fastIdx = getNextIdx(fastIdx);
            fastIdx = getNextIdx(fastIdx);

            document.getElementById('statusMessage').textContent =
                `Iteration ${iteration}: slow ${nodes[oldSlow].val} → ${nodes[slowIdx].val}, fast ${nodes[oldFast].val} → ${nodes[fastIdx].val}`;

            renderLinkedList();
            updateVariables();

            // Check again after move
            if (slowIdx === fastIdx) {
                finished = true;
                document.getElementById('statusMessage').className = 'status-message success';
                document.getElementById('statusMessage').textContent =
                    `✅ CYCLE DETECTED! Slow and fast pointers meet at node ${nodes[slowIdx].val}`;
                document.getElementById('stepBtn').disabled = true;
                stopAuto();
            }
        }

        function toggleAuto() {
            if (autoInterval) {
                stopAuto();
            } else {
                document.getElementById('autoBtn').textContent = 'Pause';
                autoInterval = setInterval(() => {
                    if (finished) {
                        stopAuto();
                    } else {
                        step();
                    }
                }, 1000);
            }
        }

        function stopAuto() {
            if (autoInterval) {
                clearInterval(autoInterval);
                autoInterval = null;
            }
            document.getElementById('autoBtn').textContent = 'Auto Run';
        }

        function reset() {
            stopAuto();
            slowIdx = 0;
            fastIdx = 1;
            iteration = 0;
            finished = false;
            document.getElementById('stepBtn').disabled = false;
            document.getElementById('statusMessage').className = 'status-message';
            document.getElementById('statusMessage').textContent = 'Click "Step" or "Auto Run" to start visualization';
            init();
        }

        init();
    </script>
</body>
</html>