algorithm-neet-code-top-150-python-visual/visual/0876_middle_of_the_linked_list.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 876: Middle of the Linked List - 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">#876</span> Middle of the Linked List</h1>
            <p>Given the head of a singly linked list, return the middle node. If two middle nodes, return the second one.</p>
            <div class="problem-meta">
                <span class="meta-tag">🔗 Linked List</span>
                <span class="meta-tag">👉👉 Two Pointers</span>
                <span class="meta-tag">⏱️ O(n)</span>
                <span class="meta-tag">💾 O(1)</span>
            </div>
            <div class="file-ref">
                📄 Python: <code>python/0876_middle_of_the_linked_list/0876_middle_of_the_linked_list.py</code>
            </div>
            </div>

        <div class="explanation-panel">
            <h4>🧠 How It Works (Layman's Terms)</h4>
            <p>Imagine two runners on a track. One runs twice as fast:</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>When fast reaches end:</strong> Slow is at the middle!</li>
                <li><strong>Why?</strong> Fast travels 2x distance, so slow is at halfway</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>
                <select id="listSize" style="padding: 8px; border-radius: 5px; margin-left: 10px;">
                    <option value="5">5 nodes</option>
                    <option value="6" selected>6 nodes</option>
                    <option value="7">7 nodes</option>
                </select>
            </div>

            <div class="status-message" id="statusMessage">
                Click "Step" to find the middle node using slow/fast pointers
            </div>

            <div class="variable-display">
                <div class="variable-box">
                    <div class="variable-name">🐢 Slow</div>
                    <div class="variable-value" id="slowVal" style="color: #4caf50;">1</div>
                </div>
                <div class="variable-box">
                    <div class="variable-name">🐇 Fast</div>
                    <div class="variable-value" id="fastVal" style="color: #ff5722;">1</div>
                </div>
                <div class="variable-box">
                    <div class="variable-name">Middle</div>
                    <div class="variable-value" id="middleVal">?</div>
                </div>
            </div>

            <div class="array-section">
                <div class="array-label">🔗 Linked List:</div>
                <div id="listContainer" class="linked-list-container"></div>
            </div>

            <div style="margin-top: 20px; text-align: center;">
                <svg id="pointerViz" width="100%" height="80"></svg>
            </div>
        </div>

        <div class="code-section">
            <h3>💻 Python Solution</h3>
            <div class="code-block">
                <pre>from typing import Optional


"""
LeetCode Middle of the Linked List

Problem from LeetCode: https://leetcode.com/problems/middle-of-the-linked-list/

Description:
Given the head of a singly linked list, return the middle node of the linked list.
If there are two middle nodes, return the second middle node.

Example 1:
Input: head = [1,2,3,4,5]
Output: [3,4,5]
Explanation: The middle node of the list is node 3.

Example 2:
Input: head = [1,2,3,4,5,6]
Output: [4,5,6]
Explanation: Since the list has two middle nodes with values 3 and 4, we return the second one.
"""

class ListNode:

    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next


class Solution:

    def middle_node(self, head: Optional[ListNode]) -&gt;Optional[ListNode]:
        """
        Return the middle node of a linked list.
        If there are two middle nodes, return the second middle node.

        Args:
            head: Head of the linked list

        Returns:
            ListNode: The middle node of the linked list
        """
        slow = head
        fast = head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        return slow

    def middleNode_array(self, head: Optional[ListNode]) -&gt;Optional[ListNode]:
        """
        Find the middle node using an array.

        Args:
            head: Head of the linked list

        Returns:
            ListNode: The middle node of the linked list
        """
        nodes = []
        current = head
        while current:
            nodes.append(current)
            current = current.next
        return nodes[len(nodes) // 2]

    def middleNode_count(self, head: Optional[ListNode]) -&gt;Optional[ListNode]:
        """
        Find the middle node by counting nodes.

        Args:
            head: Head of the linked list

        Returns:
            ListNode: The middle node of the linked list
        """
        count = 0
        current = head
        while current:
            count += 1
            current = current.next
        middle = count // 2
        current = head
        for _ in range(middle):
            current = current.next
        return current


if __name__ == '__main__':
    # Example usage based on LeetCode sample
    solution = Solution()

    # Example 1: Create the linked list [1,2,3,4,5]
    head1 = ListNode(1)
    head1.next = ListNode(2)
    head1.next.next = ListNode(3)
    head1.next.next.next = ListNode(4)
    head1.next.next.next.next = ListNode(5)

    result1 = solution.middle_node(head1)

    # Print the result
    print("Example 1 result:")
    output1 = []
    while result1:
        output1.append(result1.val)
        result1 = result1.next
    print(output1)  # Expected output: [3,4,5]

    # Example 2: Create the linked list [1,2,3,4,5,6]
    head2 = ListNode(1)
    head2.next = ListNode(2)
    head2.next.next = ListNode(3)
    head2.next.next.next = ListNode(4)
    head2.next.next.next.next = ListNode(5)
    head2.next.next.next.next.next = ListNode(6)

    result2 = solution.middle_node(head2)

    # Print the result
    print("Example 2 result:")
    output2 = []
    while result2:
        output2.append(result2.val)
        result2 = result2.next
    print(output2)  # Expected output: [4,5,6]
</pre>
            </div>
        </div>
    </div>

    <script>
        let values = [1, 2, 3, 4, 5, 6];
        let slowIdx = 0;
        let fastIdx = 0;
        let done = false;
        let autoInterval = null;

        function init() {
            const size = parseInt(document.getElementById('listSize').value);
            values = Array.from({length: size}, (_, i) => i + 1);
            renderList();
        }

        function renderList() {
            const container = document.getElementById('listContainer');
            container.innerHTML = '';

            for (let i = 0; i < values.length; i++) {
                const nodeDiv = document.createElement('div');
                nodeDiv.className = 'list-node';

                const box = document.createElement('div');
                box.className = 'node-box';
                box.textContent = values[i];
                box.id = `node-${i}`;

                if (done && i === slowIdx) {
                    box.style.background = '#4caf50';
                    box.style.transform = 'scale(1.2)';
                } else if (i === slowIdx && i === fastIdx) {
                    box.style.background = 'linear-gradient(135deg, #4caf50 50%, #ff5722 50%)';
                } else if (i === slowIdx) {
                    box.style.background = '#4caf50';
                } else if (i === fastIdx) {
                    box.style.background = '#ff5722';
                }

                nodeDiv.appendChild(box);

                if (i < values.length - 1) {
                    const arrow = document.createElement('span');
                    arrow.className = 'node-arrow';
                    arrow.textContent = '→';
                    nodeDiv.appendChild(arrow);
                }

                container.appendChild(nodeDiv);
            }

            const nullSpan = document.createElement('span');
            nullSpan.className = 'null-node';
            nullSpan.textContent = '→ null';
            container.appendChild(nullSpan);

            renderPointers();
        }

        function renderPointers() {
            const svg = d3.select("#pointerViz");
            svg.selectAll("*").remove();

            const container = document.getElementById('listContainer');
            const width = container.scrollWidth;
            svg.attr("viewBox", `0 0 ${width} 80`);

            const nodeWidth = 60;
            const gap = 8;
            const offset = 30;

            // Slow pointer (turtle)
            const slowX = offset + slowIdx * (nodeWidth + gap + 20);
            svg.append("text")
                .attr("x", slowX)
                .attr("y", 30)
                .attr("text-anchor", "middle")
                .attr("font-size", "24px")
                .text("🐢");
            svg.append("text")
                .attr("x", slowX)
                .attr("y", 55)
                .attr("text-anchor", "middle")
                .attr("fill", "#4caf50")
                .attr("font-size", "12px")
                .attr("font-weight", "bold")
                .text("slow");

            // Fast pointer (rabbit) - only if not past end
            if (fastIdx < values.length) {
                const fastX = offset + fastIdx * (nodeWidth + gap + 20);
                svg.append("text")
                    .attr("x", fastX)
                    .attr("y", 30)
                    .attr("text-anchor", "middle")
                    .attr("font-size", "24px")
                    .text("🐇");
                svg.append("text")
                    .attr("x", fastX)
                    .attr("y", 55)
                    .attr("text-anchor", "middle")
                    .attr("fill", "#ff5722")
                    .attr("font-size", "12px")
                    .attr("font-weight", "bold")
                    .text("fast");
            }
        }

        function step() {
            if (done) return;

            // Check if fast can move
            if (fastIdx >= values.length - 1 || fastIdx + 1 >= values.length) {
                done = true;
                document.getElementById('middleVal').textContent = values[slowIdx];
                document.getElementById('statusMessage').className = 'status-message success';
                document.getElementById('statusMessage').textContent =
                    `✅ Found middle! Node with value ${values[slowIdx]} at index ${slowIdx}`;
                document.getElementById('stepBtn').disabled = true;
                stopAuto();
                renderList();
                return;
            }

            // Move pointers
            slowIdx += 1;
            fastIdx += 2;

            document.getElementById('slowVal').textContent = values[slowIdx];
            document.getElementById('fastVal').textContent =
                fastIdx < values.length ? values[fastIdx] : 'null';

            document.getElementById('statusMessage').textContent =
                `slow moves to ${values[slowIdx]}, fast moves to ${fastIdx < values.length ? values[fastIdx] : 'null'}`;

            renderList();

            // Check if we should stop
            if (fastIdx >= values.length - 1) {
                done = true;
                document.getElementById('middleVal').textContent = values[slowIdx];
                document.getElementById('statusMessage').className = 'status-message success';
                document.getElementById('statusMessage').textContent =
                    `✅ Fast reached end! Middle is ${values[slowIdx]}`;
                document.getElementById('stepBtn').disabled = true;
                stopAuto();
            }
        }

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

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

        function reset() {
            stopAuto();
            slowIdx = 0;
            fastIdx = 0;
            done = false;
            document.getElementById('stepBtn').disabled = false;
            document.getElementById('statusMessage').className = 'status-message';
            document.getElementById('statusMessage').textContent =
                'Click "Step" to find the middle node using slow/fast pointers';
            document.getElementById('middleVal').textContent = '?';
            init();
            document.getElementById('slowVal').textContent = values[0];
            document.getElementById('fastVal').textContent = values[0];
        }

        document.getElementById('listSize').addEventListener('change', reset);
        init();
        document.getElementById('slowVal').textContent = values[0];
        document.getElementById('fastVal').textContent = values[0];
    </script>
</body>
</html>