1634. Add Two Polynomials Represented as Linked Lists

A polynomial linked list is a special type of linked list where every node represents a term in a polynomial expression.

Each node has three attributes:

• coefficient: an integer representing the number multiplier of the term. The coefficient of the term 9x4 is 9.
• power: an integer representing the exponent. The power of the term 9x4 is 4.
• next: a pointer to the next node in the list, or null if it is the last node of the list.

For example, the polynomial 5x3 + 4x - 7 is represented by the polynomial linked list illustrated below:

The polynomial linked list must be in its standard form: the polynomial must be in strictly descending order by its power value. Also, terms with a coefficient of 0 are omitted.

Given two polynomial linked list heads, poly1 and poly2, add the polynomials together and return the head of the sum of the polynomials.

PolyNode format:

The input/output format is as a list of n nodes, where each node is represented as its [coefficient, power]. For example, the polynomial 5x3 + 4x - 7 would be represented as: [[5,3],[4,1],[-7,0]].

/**
 * Definition for polynomial singly-linked list.
 * struct PolyNode {
 *     int coefficient, power;
 *     PolyNode *next;
 *     PolyNode(): coefficient(0), power(0), next(nullptr) {};
 *     PolyNode(int x, int y): coefficient(x), power(y), next(nullptr) {};
 *     PolyNode(int x, int y, PolyNode* next): coefficient(x), power(y), next(next) {};
 * };
 */

class Solution {
public:
    PolyNode* addPoly(PolyNode* poly1, PolyNode* poly2) {
        PolyNode* dummy = new PolyNode(-1,-1);
        PolyNode* tail = dummy;
        while(poly1 and poly2) {
            if(poly1->power == poly2->power) {
                poly1->coefficient += poly2->coefficient;
                if(poly1->coefficient != 0) {
                    tail->next = poly1;
                    tail = tail->next;
                }
                poly1 = poly1->next;
                poly2 = poly2->next;
            } else if(poly1->power > poly2->power) {
                tail->next = poly1;
                poly1 = poly1->next;
                tail = tail->next;
            } else {
                tail->next = poly2;
                poly2 = poly2->next;
                tail = tail->next;
            }
            tail->next = nullptr;
        }
        if(poly1) tail->next = poly1;
        else if(poly2) tail->next = poly2;
        
        return dummy->next;
    }
};