leetcode 401. Binary Watch

class Solution {
public:
vector<string> readBinaryWatch(int num) {
vector<string> res[11];
vector<string> ans[11];
vector<string> result;
if(num > 10 || num < 0) return result;
for(int i = 0;i <= 11; i++){
int j = i, c = 0;
while(j){
if(j&1) c++;
j/=2;
}
string x="";
j = i;
do{
x = char('0'+j%10)+x;
j/=10;
}while(j>0);
res[c].push_back(x);
}
for(int i = 0;i <= 59;i++){
int j = i, c = 0;
while(j){
if(j&1)c++;
j/=2;
}

string x="";
j = i;
for(int k = 0;k < 2; k++){
x = char('0'+j%10)+x;
j/=10;
}
ans[c].push_back(x);
}
for(int i = 0;i <= num; i++){
string x="";
for(int j = 0;j < res[i].size(); j++){
for(int k = 0;k < ans[num-i].size(); k++){
x = res[i][j]+":"+ans[num-i][k];
result.push_back(x);
}
}
}
return result;
}
};