In-place Reversal (Linked List)

Key Signals & Indicators

• Problem requires reversing list order, or sublists
• Reversing groups (e.g. reverse k nodes)
• Strict auxiliary space constraint O(1)

// C++ In-place Reversal Template
struct ListNode {
    int val;
    ListNode* next;
};

ListNode* reverseList(ListNode* head) {
    ListNode* prev = nullptr;
    ListNode* curr = head;
    
    while (curr != nullptr) {
        ListNode* nextNode = curr->next; // Store next node
        curr->next = prev;               // Reverse node reference
        prev = curr;                     // Shift pointers forward
        curr = nextNode;
    }
    return prev; // New head pointer
}