rt，代码：

#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 5;

int n, a[N], vis[N], use[N], ans;

int main() {
	cin >> n;
	for (int i = 1; i <= n; i++) {
		cin >> a[i];
		vis[a[i]]++;
		use[a[i]]++;
	}
	sort(a + 1, a + n + 1);
	int m = unique(a + 1, a + n + 1) - a - 1;
	for (int i = 1; i <= m; i++) {
		int c = vis[a[i]];
		for (int j = i + 1; j <= m; j++) {
			if (!use[a[j]]) {
				continue;
			}
			if (vis[a[j]] >= c) {
				use[a[j]] -= c;
				c = 0;
			} else {
				use[a[j]] = 0;
				c -= vis[a[j]];
				vis[a[i]] -= vis[a[j]];
			}
			if (c == 0) {
				vis[a[i]] = 0;
				break;
			}
		}
	}
	for (int i = 1; i <= m; i++) {
		ans += vis[a[i]];
	}
	cout << ans;
	return 0;
}

WA on 16 18 20，问问哪错了？