解法一：双指针

双指针遍历两个多项式，按照指数大小求和加入最终结果 ans 。注意处理和为0的情况和输出格式。

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
public class Main {
    private static float EXP = 1e-5f;
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        String[] input = reader.readLine().split(" ");
        int lenA = Integer.parseInt(input[0]);
        ArrayList<Item> A = new ArrayList<>(lenA);
        for (int i = 1; i <= lenA; ++i) {
            int exponent = Integer.parseInt(input[i * 2 - 1]);
            float coef = Float.parseFloat(input[i * 2]);
            A.add(new Item(exponent, coef));
        }
        input = reader.readLine().split(" ");
        int lenB = Integer.parseInt(input[0]);
        ArrayList<Item> B = new ArrayList<>(lenB);
        for (int i = 1; i <= lenB; ++i) {
            int exponent = Integer.parseInt(input[i * 2 - 1]);
            float coef = Float.parseFloat(input[i * 2]);
            B.add(new Item(exponent, coef));
        }
        ArrayList<Item> ans = new ArrayList<>(lenA);
        int p1 = 0, p2 = 0;
        while ((p1 < lenA) && (p2 < lenB)) {
            int flag = A.get(p1).exponent - B.get(p2).exponent;
            if (flag > 0) {
                ans.add(A.get(p1++));
            } else if (flag < 0) {
                ans.add(B.get(p2++));
            } else {
                float sum = A.get(p1).coef + B.get(p2).coef;
                if (!equalZero(sum)) {
                    ans.add(new Item(A.get(p1).exponent, sum));
                }
                ++p1;
                ++p2;
            }
        }
        while (p1 < lenA) {
            if (!equalZero(A.get(p1).coef)) {
                ans.add(A.get(p1++));
            }
        }
        while (p2 < lenB) {
            if (!equalZero(B.get(p2).coef)) {
                ans.add(B.get(p2++));
            }
        }
        System.out.print(ans.size());
        for (Item i : ans) {
            System.out.printf(" %d %.1f", i.exponent, i.coef);
        }
    }
    private static boolean equalZero(float a) {
        if (Math.abs(a) < EXP) {
            return true;
        }
        return false;
    }
}
class Item {
    int exponent;
    float coef;
    Item() {
        coef = 0.0f;
    }
    Item(int exponent, float coef) {
        this.exponent = exponent;
        this.coef = coef;
    }
}