解法一：归并

根据两个数组长度算出中位数的位置，然后进行归并。

#include <bits/stdc++.h>
#define Long long long
using namespace std;
Long A[200005];
Long B[200005];
int main() {
    int lenA, lenB;
    cin >> lenA;
    for (int i = 0; i < lenA; ++i) {
        cin >> A[i];
    }
    A[lenA] = 0x7fffffffffffffff;
    cin >> lenB;
    for (int i = 0; i < lenB; ++i) {
        cin >> B[i];
    }
    B[lenB] = 0x7fffffffffffffff;
    int pos = (lenA + lenB - 1) / 2;
    int i = 0;
    int j = 0;
    while (pos > 0) {
        if (A[i] < B[j]) {
            ++i;
        } else {
            ++j;
        }
        --pos;
    }
    if (A[i] < B[j]) {
        cout << A[i] << '\n';
    } else {
        cout << B[j] << '\n';
    }
}