Here is my accepted java solution:

```
public int[] twoSum(int[] nums, int target) {
for(int i=0; i<nums.length; i++){
for(int j=i+1; j<nums.length; j++){
if(nums[i]+nums[j] == target){
return new int[]{i, j};
}
}
}
return null;
}
```

Any suggestion to convert to O(n)?

Thank...