对于 5 6 1，如果采用res += j - mid，当 5 6 和 1 合并时，只会把 <5, 1> 算上，<6, 1>加不上了

采用res += j - mid时，想要让6对结果产生贡献，需要确保6能参与到比较当中，比如 4 6 1 5 ，4 6 和 1 5 合并

简单来说，就是res += j - mid有些情况下会算不全

牛逼，谢谢大佬👍🏻👍🏻👍🏻

大佬，我想到了一种可以解决这个问题的方法了

import java.io.*;

class Read {
    StreamTokenizer st;

    public Read() {
        st = new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in)));
    }

    public int nextInt() throws IOException {
        st.nextToken();
        return (int)st.nval;
    }

    public String next() throws IOException {
        st.nextToken();
        return st.sval;
    }
}

public class Main {
    public static void main(String[] args) throws IOException {
        Read sc = new Read();
        int n = sc.nextInt();
        int[] q = new int[n];
        for (int i = 0; i < n; i++) {
            q[i] = sc.nextInt();
        }
        System.out.println(merge_sort(q, 0, n - 1));
    }

    public static long merge_sort(int[] q, int l, int r) {
        if (l >= r) return 0;

        int mid = l + r >> 1;
        long res = merge_sort(q, l, mid) + merge_sort(q, mid + 1, r);
        for (int i = l, j = mid + 1; i <= mid; i++) {
            while (j <= r && q[i] > q[j]) j++;
            res += j - (mid + 1L);
        }

        int i = l, j = mid + 1, k = 0;
        int[] tmp = new int[r - l + 1];
        while (i <= mid && j <= r) {
            if (q[i] <= q[j]) tmp[k++] = q[i++];
            else tmp[k++] = q[j++];
        }
        while (i <= mid) tmp[k++] = q[i++];
        while (j <= r) tmp[k++] = q[j++];

        for (i = l, j = 0; i <= r; i++, j++) {
            q[i] = tmp[j];
        }
        return res;
    }
}