我的前16个灰阶排序有误帮忙查下呗

#include<bits/stdc++.h>
using namespace std;
int n;
int a[1005][1005];
int qqt(char x,char y){
	int sum=0;
	if(x<='9') sum=sum+((x-'0')*16);
	else if(x>'9'&&x<='Z') sum=sum+((x-'A'+10)*16);
	if(y<='9') sum=sum+(y-'0');
	else if(y>'9'&&y<='Z') sum=sum+(y-'A'+10);
	return sum;
}
//16进制转10进制 
struct qwe{
	int cnt;//出现次数 
	int xuhao;//序号 
	char num[2];//读入时的字符 
}sk[5000];
bool cmp(qwe a,qwe b){
	if(a.cnt!=b.cnt) return a.cnt<b.cnt;
	else if(a.cnt==b.cnt) return a.xuhao>b.xuhao;
}
int main(){
	scanf("%d",&n);
	for(int i=1;i<=n;i++){
		char q[55];
		scanf("%s",q);
		for(int j=0;j<strlen(q);j+=2){
			a[i][j]=qqt(q[j-1],q[j]);
			sk[a[i][j]].cnt++;
			sk[a[i][j]].xuhao=a[i][j];
			sk[a[i][j]].num[0]=q[j-1];
			sk[a[i][j]].num[1]=q[j];
		}
	}
	sort(sk,sk+256,cmp);
	for(int i=0;i<=14;i++){
		printf("%c%c",sk[i].num[0],sk[i].num[1]);
	}
	return 0;
}