algorithm-neet-code-top-150-python-visual/visual/0287_find_duplicate_number.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>287 - Find the Duplicate Number</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">#287</span> Find the Duplicate Number</h1>
            <p>
                Given an array containing n+1 integers where each integer is between 1 and n,
                find the duplicate using Floyd's Cycle Detection (Tortoise and Hare) algorithm
                in O(1) space without modifying the array.
            </p>

            <h3>Key Insight:</h3>
            <p>
                Treat array indices as nodes and values as next pointers. The duplicate creates a cycle!
            </p>
            <div class="problem-meta">
                <span class="meta-tag">📊 Array</span>
                <span class="meta-tag">⏱️ O(n)</span>
            </div>
            <div class="file-ref">
                📄 Python: <code>python/0287_find_the_duplicate_number/0287_find_the_duplicate_number.py</code>
            </div>

        </div>

        <div class="explanation-panel">
            <h4>🧠 How It Works (Layman's Terms)</h4>
            <p>This algorithm solves the problem <strong>step by step</strong>:</p>
            <ul>
                <li><strong>Understand:</strong> Parse the input data</li>
                <li><strong>Process:</strong> Apply the core logic</li>
                <li><strong>Optimize:</strong> Use efficient data structures</li>
                <li><strong>Return:</strong> Output the computed result</li>
            </ul>
        </div>


        <section class="visualization-section">
            <h3>🎬 Step-by-Step Visualization</h3>
            <div class="controls">
                <button id="stepBtn" class="btn">Step</button>
                <button id="autoBtn" class="btn btn-success">Auto Run</button>
                <button id="resetBtn" class="btn btn-danger">Reset</button>
            </div>
            <div class="status" id="status">Find duplicate using Floyd's Cycle Detection</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">findDuplicate</span>(nums):
    <span class="comment"># Phase 1: Find intersection point</span>
    slow = nums[<span class="number">0</span>]
    fast = nums[nums[<span class="number">0</span>]]
    <span class="keyword">while</span> slow != fast:
        slow = nums[slow]
        fast = nums[nums[fast]]

    <span class="comment"># Phase 2: Find cycle entrance (the duplicate)</span>
    slow = <span class="number">0</span>
    <span class="keyword">while</span> slow != fast:
        slow = nums[slow]
        fast = nums[fast]

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

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

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

        const nums = [1, 3, 4, 2, 2]; // The duplicate is 2

        let phase = 1;
        let slow = 0;
        let fast = 0;
        let steps = [];
        let currentStep = 0;
        let isRunning = false;
        let found = false;

        function generateSteps() {
            steps = [];
            let s = nums[0];
            let f = nums[nums[0]];
            steps.push({ phase: 1, slow: s, fast: f, message: "Phase 1: slow = nums[0], fast = nums[nums[0]]" });

            // Phase 1: Find intersection
            while (s !== f) {
                s = nums[s];
                f = nums[nums[f]];
                steps.push({ phase: 1, slow: s, fast: f, message: `Phase 1: slow → ${s}, fast → ${f}` });
            }

            steps.push({ phase: 1.5, slow: s, fast: f, message: `Intersection found at ${s}! Starting Phase 2...` });

            // Phase 2: Find entrance
            s = 0;
            steps.push({ phase: 2, slow: s, fast: f, message: "Phase 2: Reset slow to 0" });

            while (s !== f) {
                s = nums[s];
                f = nums[f];
                steps.push({ phase: 2, slow: s, fast: f, message: `Phase 2: slow → ${s}, fast → ${f}` });
            }

            steps.push({ phase: 3, slow: s, fast: f, duplicate: s, message: `Found duplicate: ${s}!` });
        }

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

            const currentPhase = step ? step.phase : 1;
            const slowPos = step ? step.slow : 0;
            const fastPos = step ? step.fast : 0;

            // Draw array view
            svg.append("text")
                .attr("x", 50)
                .attr("y", 40)
                .attr("font-size", "16px")
                .attr("font-weight", "bold")
                .attr("fill", "#1e293b")
                .text("Array View:");

            const boxSize = 60;
            const startX = 80;
            const startY = 60;

            nums.forEach((val, idx) => {
                svg.append("rect")
                    .attr("x", startX + idx * (boxSize + 10))
                    .attr("y", startY)
                    .attr("width", boxSize)
                    .attr("height", boxSize)
                    .attr("rx", 8)
                    .attr("fill", () => {
                        if (idx === slowPos && idx === fastPos) return "#fef3c7";
                        if (idx === slowPos) return "#dbeafe";
                        if (idx === fastPos) return "#d1fae5";
                        return "#f8fafc";
                    })
                    .attr("stroke", () => {
                        if (idx === slowPos && idx === fastPos) return "#f59e0b";
                        if (idx === slowPos) return "#3b82f6";
                        if (idx === fastPos) return "#10b981";
                        return "#94a3b8";
                    })
                    .attr("stroke-width", (idx === slowPos || idx === fastPos) ? 3 : 2);

                svg.append("text")
                    .attr("x", startX + idx * (boxSize + 10) + boxSize / 2)
                    .attr("y", startY - 10)
                    .attr("text-anchor", "middle")
                    .attr("font-size", "12px")
                    .attr("fill", "#64748b")
                    .text(`idx=${idx}`);

                svg.append("text")
                    .attr("x", startX + idx * (boxSize + 10) + boxSize / 2)
                    .attr("y", startY + boxSize / 2 + 6)
                    .attr("text-anchor", "middle")
                    .attr("font-size", "20px")
                    .attr("font-weight", "bold")
                    .attr("fill", "#1e293b")
                    .text(val);
            });

            // Draw linked list / cycle view
            svg.append("text")
                .attr("x", 50)
                .attr("y", 200)
                .attr("font-size", "16px")
                .attr("font-weight", "bold")
                .attr("fill", "#1e293b")
                .text("Linked List View (index → nums[index]):");

            // Node positions for the cycle
            const nodePositions = {
                0: { x: 100, y: 300 },
                1: { x: 220, y: 300 },
                3: { x: 340, y: 300 },
                4: { x: 460, y: 340 },
                2: { x: 340, y: 380 }
            };

            // Draw arrows first
            const arrows = [
                [0, nums[0]],
                [1, nums[1]],
                [2, nums[2]],
                [3, nums[3]],
                [4, nums[4]]
            ];

            // Arrow marker
            svg.append("defs").append("marker")
                .attr("id", "arrow")
                .attr("viewBox", "0 -5 10 10")
                .attr("refX", 25)
                .attr("refY", 0)
                .attr("markerWidth", 6)
                .attr("markerHeight", 6)
                .attr("orient", "auto")
                .append("path")
                .attr("d", "M0,-5L10,0L0,5")
                .attr("fill", "#64748b");

            arrows.forEach(([from, to]) => {
                const fromPos = nodePositions[from];
                const toPos = nodePositions[to];
                if (fromPos && toPos) {
                    const dx = toPos.x - fromPos.x;
                    const dy = toPos.y - fromPos.y;
                    const len = Math.sqrt(dx * dx + dy * dy);

                    // For the cycle edge (2 → 4 or 4 → 2), draw curved
                    if ((from === 2 && to === 4) || (from === 4 && to === 2)) {
                        const midX = (fromPos.x + toPos.x) / 2;
                        const midY = (fromPos.y + toPos.y) / 2 + 40;
                        svg.append("path")
                            .attr("d", `M ${fromPos.x} ${fromPos.y} Q ${midX} ${midY} ${toPos.x} ${toPos.y}`)
                            .attr("fill", "none")
                            .attr("stroke", from === 4 ? "#ef4444" : "#64748b")
                            .attr("stroke-width", from === 4 ? 3 : 2)
                            .attr("marker-end", "url(#arrow)");
                    } else {
                        svg.append("line")
                            .attr("x1", fromPos.x)
                            .attr("y1", fromPos.y)
                            .attr("x2", toPos.x)
                            .attr("y2", toPos.y)
                            .attr("stroke", "#64748b")
                            .attr("stroke-width", 2)
                            .attr("marker-end", "url(#arrow)");
                    }
                }
            });

            // Draw nodes
            Object.entries(nodePositions).forEach(([idx, pos]) => {
                const i = parseInt(idx);
                const isSlow = i === slowPos;
                const isFast = i === fastPos;
                const isDuplicate = step && step.duplicate === i;

                svg.append("circle")
                    .attr("cx", pos.x)
                    .attr("cy", pos.y)
                    .attr("r", 25)
                    .attr("fill", () => {
                        if (isDuplicate) return "#fef3c7";
                        if (isSlow && isFast) return "#fef3c7";
                        if (isSlow) return "#dbeafe";
                        if (isFast) return "#d1fae5";
                        return "#f8fafc";
                    })
                    .attr("stroke", () => {
                        if (isDuplicate) return "#f59e0b";
                        if (isSlow) return "#3b82f6";
                        if (isFast) return "#10b981";
                        return "#64748b";
                    })
                    .attr("stroke-width", (isSlow || isFast || isDuplicate) ? 3 : 2);

                svg.append("text")
                    .attr("x", pos.x)
                    .attr("y", pos.y + 5)
                    .attr("text-anchor", "middle")
                    .attr("font-size", "18px")
                    .attr("font-weight", "bold")
                    .attr("fill", "#1e293b")
                    .text(i);
            });

            // Highlight cycle
            svg.append("text")
                .attr("x", 400)
                .attr("y", 430)
                .attr("text-anchor", "middle")
                .attr("font-size", "14px")
                .attr("fill", "#ef4444")
                .text("← Cycle (caused by duplicate value 2)");

            // Phase indicator
            svg.append("text")
                .attr("x", 700)
                .attr("y", 250)
                .attr("font-size", "16px")
                .attr("font-weight", "bold")
                .attr("fill", "#1e293b")
                .text(`Phase: ${currentPhase < 2 ? 1 : currentPhase === 3 ? "Done" : 2}`);

            svg.append("text")
                .attr("x", 700)
                .attr("y", 280)
                .attr("font-size", "14px")
                .attr("fill", currentPhase < 2 ? "#3b82f6" : "#64748b")
                .text("1: Find intersection");

            svg.append("text")
                .attr("x", 700)
                .attr("y", 305)
                .attr("font-size", "14px")
                .attr("fill", currentPhase >= 2 ? "#10b981" : "#64748b")
                .text("2: Find cycle entrance");

            // Legend
            const legend = svg.append("g").attr("transform", `translate(700, 340)`);

            legend.append("circle").attr("cx", 10).attr("cy", 0).attr("r", 10).attr("fill", "#dbeafe").attr("stroke", "#3b82f6");
            legend.append("text").attr("x", 30).attr("y", 5).attr("font-size", "12px").text("Slow pointer");

            legend.append("circle").attr("cx", 10).attr("cy", 30).attr("r", 10).attr("fill", "#d1fae5").attr("stroke", "#10b981");
            legend.append("text").attr("x", 30).attr("y", 35).attr("font-size", "12px").text("Fast pointer");

            legend.append("circle").attr("cx", 10).attr("cy", 60).attr("r", 10).attr("fill", "#fef3c7").attr("stroke", "#f59e0b");
            legend.append("text").attr("x", 30).attr("y", 65).attr("font-size", "12px").text("Both / Duplicate");

            // Pointer values
            svg.append("text")
                .attr("x", 700)
                .attr("y", 450)
                .attr("font-size", "14px")
                .attr("fill", "#3b82f6")
                .text(`Slow: ${slowPos}`);

            svg.append("text")
                .attr("x", 780)
                .attr("y", 450)
                .attr("font-size", "14px")
                .attr("fill", "#10b981")
                .text(`Fast: ${fastPos}`);
        }

        function reset() {
            generateSteps();
            currentStep = 0;
            isRunning = false;
            found = false;
            document.getElementById("autoBtn").textContent = "Auto Run";
            document.getElementById("status").textContent = "Find duplicate using Floyd's Cycle Detection";
            render(null);
        }

        function step() {
            if (currentStep < steps.length) {
                const s = steps[currentStep];
                render(s);
                document.getElementById("status").textContent = s.message;
                currentStep++;
            } else {
                document.getElementById("status").textContent = "Algorithm complete! The duplicate is 2.";
            }
        }

        async function autoRun() {
            if (isRunning) {
                isRunning = false;
                document.getElementById("autoBtn").textContent = "Auto Run";
                return;
            }

            isRunning = true;
            document.getElementById("autoBtn").textContent = "Pause";

            while (currentStep < steps.length && isRunning) {
                step();
                await new Promise(r => setTimeout(r, 1000));
            }

            isRunning = false;
            document.getElementById("autoBtn").textContent = "Auto Run";
        }

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

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