algorithm-neet-code-top-150-python-visual/visual/0424_longest_repeating_character.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>Longest Repeating Character Replacement - LeetCode 424</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">#424</span> Longest Repeating Character Replacement</h1>
            <p>Given a string and k replacements allowed, find the longest substring with all same characters. The trick: keep track of the most frequent character in the window!</p>
            <div class="problem-meta">
                <span class="meta-tag">🪟 Sliding Window</span>
                <span class="meta-tag">🔤 String</span>
                <span class="meta-tag">⏱️ O(n)</span>
            </div>
            <div class="file-ref">
                📄 Python: <a href="../python/0424_longest_repeating_character_replacement/0424_longest_repeating_character_replacement.py">0424_longest_repeating_character_replacement.py</a>
            </div>
            </div>

        <div class="explanation-panel">
            <h4>💡 How It Works (Layman's Terms)</h4>
            <ul>
                <li><strong>Goal:</strong> Find the longest substring where all characters are the same, using at most k replacements</li>
                <li><strong>Key insight:</strong> In any valid window, we keep the most frequent character and replace the others</li>
                <li><strong>Formula:</strong> replacements_needed = window_size - max_frequency</li>
                <li><strong>Valid window:</strong> When replacements_needed ≤ k</li>
                <li><strong>Expand:</strong> Move right pointer, add character to window</li>
                <li><strong>Shrink:</strong> When we need more than k replacements, move left pointer</li>
                <li><strong>Track max:</strong> Keep track of the maximum valid window size we've seen</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">
                k = 2 (can replace up to 2 characters)
            </div>

            <div class="status-message" id="statusMessage">
                Click "Step" or "Auto Run" to find the longest substring with replacements
            </div>

            <div class="variable-display">
                <div class="variable-box">
                    <div class="variable-name">Window Size</div>
                    <div class="variable-value" id="windowVal">0</div>
                </div>
                <div class="variable-box">
                    <div class="variable-name">Max Frequency</div>
                    <div class="variable-value" id="freqVal">0</div>
                </div>
                <div class="variable-box">
                    <div class="variable-name">Replacements Needed</div>
                    <div class="variable-value" id="replaceVal">0</div>
                </div>
                <div class="variable-box">
                    <div class="variable-name">Max Length</div>
                    <div class="variable-value" id="maxVal">0</div>
                </div>
            </div>

            <div class="array-section">
                <div class="array-label">String:</div>
                <div class="array-container" id="stringContainer"></div>
            </div>

            <div class="array-section">
                <div class="array-label">Character Counts in Window:</div>
                <div id="countsContainer" style="display: flex; gap: 10px; flex-wrap: wrap;"></div>
            </div>
        </div>
        <div class="code-section">
            <h3>💻 Python Solution</h3>
            <div class="code-block">
                <pre><span class="keyword">def</span> <span class="function">characterReplacement</span>(self, s: <span class="class-name">str</span>, k: <span class="class-name">int</span>) -> int:
    char_counts = {}
    left = <span class="number">0</span>
    max_freq = <span class="number">0</span>  <span class="comment"># Most frequent char count in window</span>
    max_length = <span class="number">0</span>

    <span class="keyword">for</span> right <span class="keyword">in</span> <span class="function">range</span>(<span class="function">len</span>(s)):
        char_counts[s[right]] = char_counts.<span class="function">get</span>(s[right], <span class="number">0</span>) + <span class="number">1</span>
        max_freq = <span class="function">max</span>(max_freq, char_counts[s[right]])

        <span class="comment"># If we need more than k replacements, shrink window</span>
        <span class="keyword">if</span> (right - left + <span class="number">1</span>) - max_freq > k:
            char_counts[s[left]] -= <span class="number">1</span>
            left += <span class="number">1</span>

        max_length = <span class="function">max</span>(max_length, right - left + <span class="number">1</span>)

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

        </div>

    <script>
        const s = "AABABBA";
        const k = 2;
        let charCounts = {};
        let left = 0;
        let right = -1;
        let maxFreq = 0;
        let maxLength = 0;
        let phase = 'init';
        let autoInterval = null;

        function init() {
            renderString();
            renderCounts();
            document.getElementById('windowVal').textContent = '0';
            document.getElementById('freqVal').textContent = '0';
            document.getElementById('replaceVal').textContent = '0';
            document.getElementById('maxVal').textContent = '0';
        }

        function renderString() {
            const container = document.getElementById('stringContainer');
            container.innerHTML = '';
            s.split('').forEach((char, idx) => {
                const box = document.createElement('div');
                box.className = 'array-box';
                box.id = `char-${idx}`;
                box.style.width = '50px';
                box.innerHTML = `${char}<span class="index-label">[${idx}]</span>`;

                if (idx >= left && idx <= right) {
                    box.classList.add('current');
                }
                if (idx === right) {
                    box.classList.add('highlight');
                }
                container.appendChild(box);
            });

            // Draw window indicators
            if (right >= 0) {
                let windowIndicator = document.getElementById('windowIndicator');
                if (!windowIndicator) {
                    windowIndicator = document.createElement('div');
                    windowIndicator.id = 'windowIndicator';
                    windowIndicator.style.cssText = 'margin-top: 10px; display: flex; gap: 8px;';
                    container.parentNode.appendChild(windowIndicator);
                }
                let html = '';
                for (let i = 0; i < s.length; i++) {
                    html += '<div style="width: 50px; text-align: center; font-size: 0.9em;">';
                    if (i === left) html += '<span style="color: #ff5722; font-weight: bold;">L↑</span>';
                    else if (i === right) html += '<span style="color: #3f51b5; font-weight: bold;">R↑</span>';
                    html += '</div>';
                }
                windowIndicator.innerHTML = html;
            }
        }

        function renderCounts() {
            const container = document.getElementById('countsContainer');
            container.innerHTML = '';
            if (Object.keys(charCounts).length === 0) {
                container.innerHTML = '<span style="color: #999;">No characters in window yet</span>';
                return;
            }
            Object.entries(charCounts).sort().forEach(([char, count]) => {
                if (count > 0) {
                    const box = document.createElement('div');
                    box.className = 'variable-box';
                    box.innerHTML = `<div class="variable-name">${char}</div><div class="variable-value">${count}

        </div>`;
                    if (count === maxFreq) {
                        box.style.borderColor = '#4caf50';
                        box.style.background = '#e8f5e9';
                    }
                    container.appendChild(box);
                }
            });
        }

        function step() {
            if (phase === 'init') {
                phase = 'expanding';
                right = -1;
                document.getElementById('statusMessage').textContent =
                    'Starting sliding window. Expand right pointer to find valid windows.';
            }

            if (phase === 'expanding') {
                right++;

                if (right >= s.length) {
                    phase = 'done';
                    document.getElementById('statusMessage').textContent =
                        `✅ Done! Maximum length with ${k} replacements: ${maxLength}`;
                    document.getElementById('stepBtn').disabled = true;
                    stopAuto();
                    return;
                }

                // Add character
                charCounts[s[right]] = (charCounts[s[right]] || 0) + 1;
                maxFreq = Math.max(maxFreq, charCounts[s[right]]);

                const windowSize = right - left + 1;
                const replacementsNeeded = windowSize - maxFreq;

                document.getElementById('windowVal').textContent = windowSize;
                document.getElementById('freqVal').textContent = maxFreq;
                document.getElementById('replaceVal').textContent = replacementsNeeded;

                if (replacementsNeeded > k) {
                    // Need to shrink
                    document.getElementById('statusMessage').textContent =
                        `Window [${left},${right}]: "${s.substring(left, right + 1)}" needs ${replacementsNeeded} replacements > k=${k}. Shrinking!`;

                    charCounts[s[left]]--;
                    left++;
                } else {
                    maxLength = Math.max(maxLength, windowSize);
                    document.getElementById('maxVal').textContent = maxLength;
                    document.getElementById('statusMessage').textContent =
                        `Window [${left},${right}]: "${s.substring(left, right + 1)}" size=${windowSize}, most freq=${maxFreq}, replacements=${replacementsNeeded} ≤ k=${k}. Valid! Max=${maxLength}`;
                }

                renderString();
                renderCounts();
            }
        }

        function toggleAuto() {
            if (autoInterval) {
                stopAuto();
            } else {
                document.getElementById('autoBtn').textContent = 'Pause';
                autoInterval = setInterval(() => {
                    if (phase === 'done') {
                        stopAuto();
                    } else {
                        step();
                    }
                }, 1300);
            }
        }

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

        function reset() {
            stopAuto();
            phase = 'init';
            left = 0;
            right = -1;
            charCounts = {};
            maxFreq = 0;
            maxLength = 0;
            document.getElementById('stepBtn').disabled = false;
            document.getElementById('statusMessage').textContent =
                'Click "Step" or "Auto Run" to find the longest substring with replacements';
            const windowIndicator = document.getElementById('windowIndicator');
            if (windowIndicator) windowIndicator.remove();
            init();
        }

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