algorithm-neet-code-top-150-python-visual/visual/0091_decode_ways.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>091 - Decode Ways</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">#091</span> Decode Ways</h1>
            <p>
                Given a string of digits, count the ways to decode it as letters (A=1, B=2, ..., Z=26).
                Uses dynamic programming: dp[i] = dp[i-1] (single digit) + dp[i-2] (two digits if valid).
            </p>
            <div class="problem-meta">
                <span class="meta-tag">🔤 String</span>
                <span class="meta-tag">🧮 DP</span>
                <span class="meta-tag">⏱️ O(n)</span>
            </div>
            <div class="file-ref">
                📄 Python: <code>python/0091_decode_ways/0091_decode_ways.py</code>
            </div>

        </div>

        <div class="explanation-panel">
            <h4>🧠 How It Works (Layman's Terms)</h4>
            <p>Dynamic Programming <strong>breaks big problems into smaller ones</strong>:</p>
            <ul>
                <li><strong>Subproblems:</strong> Solve smaller versions first</li>
                <li><strong>Memoization:</strong> Cache results to avoid recalculation</li>
                <li><strong>Build up:</strong> Combine small solutions for final answer</li>
                <li><strong>State:</strong> Define what each position represents</li>
            </ul>
        </div>


        <section class="visualization-section">
            <h3>🎬 Step-by-Step Visualization</h3>
            <div class="controls">
                <button id="autoRunBtn" class="btn">▶ Auto Run</button>
                <button id="stepBtn" class="btn btn-success">Step</button>
                <button id="resetBtn" class="btn btn-danger">Reset</button>
            </div>
            <div class="status" id="status">Click Auto Run to decode "226"</div>
            <svg id="visualization"></svg>
        </section>

        <section class="code-section">
            <h3>💻 Python Solution</h3>
            <div class="code-block">
                <pre><span class="keyword">def</span> <span class="function">numDecodings</span>(s):
    <span class="keyword">if</span> <span class="keyword">not</span> s <span class="keyword">or</span> s[<span class="number">0</span>] == <span class="string">'0'</span>:
        <span class="keyword">return</span> <span class="number">0</span>

    prev2, prev1 = <span class="number">1</span>, <span class="number">1</span>

    <span class="keyword">for</span> i <span class="keyword">in</span> <span class="function">range</span>(<span class="number">1</span>, <span class="function">len</span>(s)):
        current = <span class="number">0</span>

        <span class="comment"># Single digit (1-9)</span>
        <span class="keyword">if</span> s[i] != <span class="string">'0'</span>:
            current += prev1

        <span class="comment"># Two digits (10-26)</span>
        two_digit = <span class="function">int</span>(s[i-<span class="number">1</span>:<span class="class-name">i</span>+<span class="number">1</span>])
        <span class="keyword">if</span> <span class="number">10</span> <= two_digit <= <span class="number">26</span>:
            current += prev2

        prev2, prev1 = prev1, current

    <span class="keyword">return</span> prev1</pre>
            </div>
        </section>
    </div>

    <script>
        const width = 900;
        const height = 550;

        const svg = d3.select("#visualization")
            .attr("width", width)
            .attr("height", height);

        const s = "226";
        let dp = [];
        let currentIdx = 0;
        let prev2 = 1;
        let prev1 = 1;
        let decodings = [];
        let animationTimer = null;

        // Mapping
        const charMap = {};
        for (let i = 1; i <= 26; i++) {
            charMap[i] = String.fromCharCode(64 + i);
        }

        function reset() {
            dp = [1];  // dp[0] = 1 for empty prefix
            currentIdx = 0;
            prev2 = 1;
            prev1 = 1;
            decodings = [];

            // Calculate all decodings
            calculateDecodings(s, 0, '');

            if (animationTimer) clearInterval(animationTimer);
            document.getElementById("status").textContent = `Decoding "${s}" - found ${decodings.length} possible decodings`;
            render();
        }

        function calculateDecodings(str, idx, current) {
            if (idx === str.length) {
                decodings.push(current);
                return;
            }

            // Single digit
            if (str[idx] !== '0') {
                const digit = parseInt(str[idx]);
                calculateDecodings(str, idx + 1, current + charMap[digit]);
            }

            // Two digits
            if (idx + 1 < str.length) {
                const twoDigit = parseInt(str.substring(idx, idx + 2));
                if (twoDigit >= 10 && twoDigit <= 26) {
                    calculateDecodings(str, idx + 2, current + charMap[twoDigit]);
                }
            }
        }

        function render() {
            svg.selectAll("*").remove();

            // Draw input string
            drawInput();

            // Draw DP table
            drawDPTable();

            // Draw character mapping
            drawMapping();

            // Draw possible decodings
            drawDecodings();
        }

        function drawInput() {
            svg.append("text")
                .attr("x", 30)
                .attr("y", 40)
                .attr("font-size", "14px")
                .attr("font-weight", "bold")
                .attr("fill", "#1e293b")
                .text("Input String:");

            s.split('').forEach((ch, idx) => {
                const x = 140 + idx * 50;
                const isProcessed = idx < currentIdx;
                const isCurrent = idx === currentIdx;

                svg.append("rect")
                    .attr("x", x)
                    .attr("y", 20)
                    .attr("width", 45)
                    .attr("height", 45)
                    .attr("rx", 8)
                    .attr("fill", () => {
                        if (isCurrent) return "#fef3c7";
                        if (isProcessed) return "#d1fae5";
                        return "#f8fafc";
                    })
                    .attr("stroke", () => {
                        if (isCurrent) return "#f59e0b";
                        if (isProcessed) return "#10b981";
                        return "#94a3b8";
                    })
                    .attr("stroke-width", isCurrent ? 3 : 2);

                svg.append("text")
                    .attr("x", x + 22)
                    .attr("y", 50)
                    .attr("text-anchor", "middle")
                    .attr("font-size", "20px")
                    .attr("font-weight", "bold")
                    .attr("fill", "#1e293b")
                    .text(ch);

                // Index label
                svg.append("text")
                    .attr("x", x + 22)
                    .attr("y", 80)
                    .attr("text-anchor", "middle")
                    .attr("font-size", "11px")
                    .attr("fill", "#64748b")
                    .text(`i=${idx}`);
            });
        }

        function drawDPTable() {
            const startX = 30;
            const startY = 130;

            svg.append("text")
                .attr("x", startX)
                .attr("y", startY)
                .attr("font-size", "14px")
                .attr("font-weight", "bold")
                .attr("fill", "#1e293b")
                .text("DP Values (ways to decode s[0:i]):");

            // Draw dp values
            for (let i = 0; i <= s.length; i++) {
                const x = startX + i * 70;
                const y = startY + 25;
                const value = i < dp.length ? dp[i] : "?";
                const isCalculated = i < dp.length;

                svg.append("rect")
                    .attr("x", x)
                    .attr("y", y)
                    .attr("width", 60)
                    .attr("height", 40)
                    .attr("rx", 6)
                    .attr("fill", isCalculated ? "#dbeafe" : "#f8fafc")
                    .attr("stroke", isCalculated ? "#3b82f6" : "#94a3b8")
                    .attr("stroke-width", 2);

                svg.append("text")
                    .attr("x", x + 30)
                    .attr("y", y - 5)
                    .attr("text-anchor", "middle")
                    .attr("font-size", "11px")
                    .attr("fill", "#64748b")
                    .text(`dp[${i}]`);

                svg.append("text")
                    .attr("x", x + 30)
                    .attr("y", y + 28)
                    .attr("text-anchor", "middle")
                    .attr("font-size", "18px")
                    .attr("font-weight", "bold")
                    .attr("fill", "#1e293b")
                    .text(value);
            }

            // Formula explanation
            svg.append("text")
                .attr("x", startX)
                .attr("y", startY + 100)
                .attr("font-size", "12px")
                .attr("fill", "#64748b")
                .text("Formula: dp[i] = dp[i-1] (if s[i-1] valid) + dp[i-2] (if s[i-2:i] in 10-26)");
        }

        function drawMapping() {
            const startX = 30;
            const startY = 260;

            svg.append("text")
                .attr("x", startX)
                .attr("y", startY)
                .attr("font-size", "14px")
                .attr("font-weight", "bold")
                .attr("fill", "#1e293b")
                .text("Character Mapping:");

            // Show relevant mappings
            const relevantMappings = [1, 2, 6, 22, 26];
            relevantMappings.forEach((num, idx) => {
                const x = startX + idx * 80;

                svg.append("rect")
                    .attr("x", x)
                    .attr("y", startY + 15)
                    .attr("width", 70)
                    .attr("height", 30)
                    .attr("rx", 5)
                    .attr("fill", "#e0e7ff")
                    .attr("stroke", "#6366f1")
                    .attr("stroke-width", 1);

                svg.append("text")
                    .attr("x", x + 35)
                    .attr("y", startY + 36)
                    .attr("text-anchor", "middle")
                    .attr("font-size", "12px")
                    .attr("fill", "#1e293b")
                    .text(`${num} → ${charMap[num]}`);
            });
        }

        function drawDecodings() {
            const startX = 30;
            const startY = 340;

            svg.append("text")
                .attr("x", startX)
                .attr("y", startY)
                .attr("font-size", "14px")
                .attr("font-weight", "bold")
                .attr("fill", "#1e293b")
                .text(`Possible Decodings (${decodings.length} ways):`);

            decodings.forEach((decoding, idx) => {
                const x = startX + (idx % 4) * 150;
                const y = startY + 25 + Math.floor(idx / 4) * 45;

                // Show the decoding with its grouping
                let grouping = getGrouping(decoding);

                svg.append("rect")
                    .attr("x", x)
                    .attr("y", y)
                    .attr("width", 140)
                    .attr("height", 35)
                    .attr("rx", 6)
                    .attr("fill", "#d1fae5")
                    .attr("stroke", "#10b981")
                    .attr("stroke-width", 2);

                svg.append("text")
                    .attr("x", x + 70)
                    .attr("y", y + 15)
                    .attr("text-anchor", "middle")
                    .attr("font-size", "14px")
                    .attr("font-weight", "bold")
                    .attr("fill", "#1e293b")
                    .text(decoding);

                svg.append("text")
                    .attr("x", x + 70)
                    .attr("y", y + 28)
                    .attr("text-anchor", "middle")
                    .attr("font-size", "10px")
                    .attr("fill", "#64748b")
                    .text(grouping);
            });

            // Time/Space complexity
            svg.append("text")
                .attr("x", startX)
                .attr("y", 500)
                .attr("font-size", "12px")
                .attr("fill", "#64748b")
                .text("Time: O(n), Space: O(1) using two variables instead of array");
        }

        function getGrouping(decoding) {
            // Reconstruct the grouping from the decoding
            let result = [];
            let remaining = s;

            for (const char of decoding) {
                const code = char.charCodeAt(0) - 64;
                const codeStr = code.toString();
                if (remaining.startsWith(codeStr)) {
                    result.push(codeStr);
                    remaining = remaining.substring(codeStr.length);
                }
            }

            return `(${result.join(' ')})`;
        }

        function step() {
            if (currentIdx >= s.length) {
                document.getElementById("status").textContent =
                    `✓ Complete! "${s}" has ${dp[dp.length - 1]} decoding ways`;
                return;
            }

            if (currentIdx === 0) {
                // First character
                if (s[0] === '0') {
                    dp.push(0);
                    document.getElementById("status").textContent =
                        `s[0]='0' is invalid. dp[1] = 0`;
                } else {
                    dp.push(1);
                    document.getElementById("status").textContent =
                        `s[0]='${s[0]}' → '${charMap[parseInt(s[0])]}'. dp[1] = 1`;
                }
                currentIdx++;
            } else {
                let current = 0;
                let explanation = [];

                // Single digit
                if (s[currentIdx] !== '0') {
                    current += dp[currentIdx];
                    explanation.push(`single '${s[currentIdx]}' → ${charMap[parseInt(s[currentIdx])]}: +dp[${currentIdx}]=${dp[currentIdx]}`);
                }

                // Two digits
                const twoDigit = parseInt(s.substring(currentIdx - 1, currentIdx + 1));
                if (twoDigit >= 10 && twoDigit <= 26) {
                    current += dp[currentIdx - 1];
                    explanation.push(`pair '${s.substring(currentIdx - 1, currentIdx + 1)}' → ${charMap[twoDigit]}: +dp[${currentIdx - 1}]=${dp[currentIdx - 1]}`);
                }

                dp.push(current);
                document.getElementById("status").textContent =
                    `dp[${currentIdx + 1}] = ${current}. ${explanation.join(', ')}`;
                currentIdx++;
            }

            render();
        }

        function autoRun() {
            if (animationTimer) {
                clearInterval(animationTimer);
                animationTimer = null;
                document.getElementById("autoRunBtn").textContent = "▶ Auto Run";
                return;
            }

            document.getElementById("autoRunBtn").textContent = "⏸ Pause";
            animationTimer = setInterval(() => {
                if (currentIdx >= s.length) {
                    clearInterval(animationTimer);
                    animationTimer = null;
                    document.getElementById("autoRunBtn").textContent = "▶ Auto Run";
                    return;
                }
                step();
            }, 1200);
        }

        document.getElementById("autoRunBtn").addEventListener("click", autoRun);
        document.getElementById("stepBtn").addEventListener("click", step);
        document.getElementById("resetBtn").addEventListener("click", reset);

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