不知道下面的代码哪里错了

/*
    \  | ^  ^  \
   -- | #    # \
   \_|         \
*/
#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <vector>

const int N = 1e6 + 10;
int t;
int n;
int a[N];
int b[N];
int cnt[N];
std::vector<int> id[N];

void solve() {
	std::cin >> n;
	for (int i = 1; i <= n; ++i) {
		cnt[i] = 0;
		id[i].clear();
	}
	for (int i = 1; i <= n; ++i) {
		std::cin >> a[i];
		++cnt[a[i]];
		id[cnt[a[i]]].push_back(i);
	}
	for (int i = 1; i <= n; ++i) {
		if (id[i].size()) {
			for (int j = 1; j < (int)id[i].size(); ++j) {
				b[id[i][j - 1]] = a[id[i][j]];
			}
			b[id[i][(int)id[i].size() - 1]] = a[id[i][0]];
		}
	}
	for (int i = 1; i <= n; ++i) {
		std::cout << b[i] << ' ';
	}
	std::cout << '\n';
}

int main() {
    std::ios::sync_with_stdio();
    std::cin.tie(0), std::cout.tie(0);
    std::cin >> t;
    while (t--) {
    	solve();
    }
    return 0;
}

输入 7 2 5 4 7 4 2

输出 2 5 4 7 4 2 7

感觉没有问题