the answer is :
class Solution {
public:
vector twoSum(vector& nums, int target) {
for(int i = 0; i < nums.size()-1; i++) {
for(int j = i+1; j< nums.size(); j++) {
if(nums[i] + nums[j] == target) {
return {i, j};
}
}
}
return {};
}
};

but submit can be accepted.
