algorithm-neet-code-top-150-python-visual/visual/0128_longest_consecutive_sequence.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 128: Longest Consecutive Sequence - 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">#128</span> Longest Consecutive Sequence</h1>
            <p>Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence. Must run in O(n) time.</p>
            <div class="problem-meta">
                <span class="meta-tag">📁 Array</span>
                <span class="meta-tag">🔤 Hash Set</span>
                <span class="meta-tag">⏱️ O(n)</span>
            </div>
            <div class="file-ref">
                📄 Python: <code>python/0128_longest_consecutive_sequence/0128_longest_consecutive_sequence.py</code>
            </div>
            </div>

        <div class="explanation-panel">
            <h4>🧠 How It Works (Layman's Terms)</h4>
            <p>Imagine you have scattered puzzle pieces numbered 1, 2, 3, 4, 100, 200. You want to find the longest chain of consecutive numbers.</p>
            <ul>
                <li><strong>Step 1:</strong> Put all numbers in a set for fast lookup</li>
                <li><strong>Step 2:</strong> For each number, check if it's the START of a sequence (no number before it)</li>
                <li><strong>Step 3:</strong> If it's a start, count how far the sequence goes (1→2→3→4...)</li>
                <li><strong>Step 4:</strong> Track the longest chain found</li>
            </ul>
            <p>The key insight: Only start counting from sequence starts (where n-1 doesn't exist). This ensures O(n) time!</p>
        </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="status-message" id="statusMessage">
                Click "Step" or "Auto Run" to start visualization
            </div>

            <div class="variable-display">
                <div class="variable-box">
                    <div class="variable-name">Current Number</div>
                    <div class="variable-value" id="currentNum">-</div>
                </div>
                <div class="variable-box">
                    <div class="variable-name">Current Streak</div>
                    <div class="variable-value" id="currentStreak">0</div>
                </div>
                <div class="variable-box">
                    <div class="variable-name">Longest Streak</div>
                    <div class="variable-value" id="longestStreak" style="color: #4caf50;">0</div>
                </div>
            </div>

            <div class="array-section">
                <div class="array-label">📥 Input Array (as Set):</div>
                <div class="array-container" id="setContainer"></div>
            </div>

            <div class="array-section">
                <div class="array-label">🔗 Current Sequence Being Built:</div>
                <div class="array-container" id="sequenceContainer">
                    <div style="color: #999; padding: 10px;">Sequence will appear here</div>
                </div>
            </div>
        </div>
        <div class="code-section">
            <h3>💻 Python Solution</h3>
            <div class="code-block">
                <pre><span class="keyword">def</span> <span class="function">longest_consecutive</span>(self, nums: <span class="class-name">List</span>[int]) -> int:
    num_set = <span class="function">set</span>(nums)
    longest_streak = <span class="number">0</span>

    <span class="keyword">for</span> num <span class="keyword">in</span> num_set:
        <span class="comment"># Only start counting if this is the start of a sequence</span>
        <span class="keyword">if</span> num - <span class="number">1</span> <span class="keyword">not</span> <span class="keyword">in</span> num_set:
            current_num = num
            current_streak = <span class="number">1</span>

            <span class="keyword">while</span> current_num + <span class="number">1</span> <span class="keyword">in</span> num_set:
                current_num += <span class="number">1</span>
                current_streak += <span class="number">1</span>

            longest_streak = <span class="function">max</span>(longest_streak, current_streak)

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

    <script>
        const nums = [100, 4, 200, 1, 3, 2];
        const numSet = new Set(nums);
        const sortedNums = Array.from(numSet).sort((a, b) => a - b);
        let setIndex = 0;
        let currentNum = null;
        let currentStreak = 0;
        let longestStreak = 0;
        let currentSequence = [];
        let phase = 'check'; // 'check' or 'extend'
        let autoInterval = null;

        function init() {
            renderSet();
            document.getElementById('currentNum').textContent = '-';
            document.getElementById('currentStreak').textContent = '0';
            document.getElementById('longestStreak').textContent = '0';
            document.getElementById('sequenceContainer').innerHTML = '<div style="color: #999; padding: 10px;">Sequence will appear here</div>';
        }

        function renderSet() {
            const container = document.getElementById('setContainer');
            container.innerHTML = '';
            sortedNums.forEach((num) => {
                const box = document.createElement('div');
                box.className = 'array-box';
                box.id = `set-${num}`;
                box.textContent = num;
                container.appendChild(box);
            });
        }

        function renderSequence() {
            const container = document.getElementById('sequenceContainer');
            container.innerHTML = '';

            if (currentSequence.length === 0) {
                container.innerHTML = '<div style="color: #999; padding: 10px;">Sequence will appear here

        </div>';
                return;
            }

            currentSequence.forEach((num, idx) => {
                if (idx > 0) {
                    const arrow = document.createElement('span');
                    arrow.textContent = '→';
                    arrow.style.fontSize = '1.5em';
                    arrow.style.color = '#4caf50';
                    container.appendChild(arrow);
                }
                const box = document.createElement('div');
                box.className = 'array-box';
                box.style.background = '#e8f5e9';
                box.style.borderColor = '#4caf50';
                box.textContent = num;
                container.appendChild(box);
            });
        }

        function clearHighlights() {
            sortedNums.forEach(num => {
                const el = document.getElementById(`set-${num}`);
                if (el) {
                    el.classList.remove('highlight', 'current', 'visited');
                    el.style.background = '';
                    el.style.borderColor = '';
                }
            });
        }

        function step() {
            if (setIndex >= sortedNums.length) {
                document.getElementById('statusMessage').className = 'status-message success';
                document.getElementById('statusMessage').textContent =
                    `✅ Done! Longest consecutive sequence has length ${longestStreak}`;
                document.getElementById('stepBtn').disabled = true;
                stopAuto();
                return;
            }

            if (phase === 'check') {
                const num = sortedNums[setIndex];
                clearHighlights();
                document.getElementById(`set-${num}`).classList.add('highlight');
                document.getElementById('currentNum').textContent = num;

                // Check if this is start of sequence (num-1 not in set)
                if (!numSet.has(num - 1)) {
                    // Start of a new sequence
                    currentNum = num;
                    currentStreak = 1;
                    currentSequence = [num];
                    renderSequence();
                    document.getElementById('currentStreak').textContent = currentStreak;
                    document.getElementById(`set-${num}`).style.background = '#e8f5e9';
                    document.getElementById(`set-${num}`).style.borderColor = '#4caf50';

                    document.getElementById('statusMessage').textContent =
                        `${num} is a sequence START (${num-1} not in set). Starting new sequence...`;

                    // Check if we can extend
                    if (numSet.has(num + 1)) {
                        phase = 'extend';
                    } else {
                        // Single number sequence
                        longestStreak = Math.max(longestStreak, currentStreak);
                        document.getElementById('longestStreak').textContent = longestStreak;
                        setIndex++;
                    }
                } else {
                    document.getElementById('statusMessage').textContent =
                        `${num} is NOT a sequence start (${num-1} exists). Skipping...`;
                    document.getElementById(`set-${num}`).style.background = '#f5f5f5';
                    setIndex++;
                }
            } else if (phase === 'extend') {
                currentNum++;
                if (numSet.has(currentNum)) {
                    currentStreak++;
                    currentSequence.push(currentNum);
                    renderSequence();
                    document.getElementById('currentNum').textContent = currentNum;
                    document.getElementById('currentStreak').textContent = currentStreak;

                    document.getElementById(`set-${currentNum}`).classList.add('highlight');
                    document.getElementById(`set-${currentNum}`).style.background = '#e8f5e9';
                    document.getElementById(`set-${currentNum}`).style.borderColor = '#4caf50';

                    document.getElementById('statusMessage').textContent =
                        `Found ${currentNum} in set! Sequence extended to length ${currentStreak}`;

                    if (!numSet.has(currentNum + 1)) {
                        // Sequence ends
                        longestStreak = Math.max(longestStreak, currentStreak);
                        document.getElementById('longestStreak').textContent = longestStreak;
                        phase = 'check';
                        setIndex++;
                    }
                } else {
                    // Should not happen in this logic, but safety
                    longestStreak = Math.max(longestStreak, currentStreak);
                    document.getElementById('longestStreak').textContent = longestStreak;
                    phase = 'check';
                    setIndex++;
                }
            }
        }

        function toggleAuto() {
            if (autoInterval) {
                stopAuto();
            } else {
                document.getElementById('autoBtn').textContent = 'Pause';
                autoInterval = setInterval(() => {
                    if (setIndex >= sortedNums.length) {
                        step();
                        stopAuto();
                    } else {
                        step();
                    }
                }, 800);
            }
        }

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

        function reset() {
            stopAuto();
            setIndex = 0;
            currentNum = null;
            currentStreak = 0;
            longestStreak = 0;
            currentSequence = [];
            phase = 'check';
            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>