Iterate the array and find largest and second largest number. Check if largest is twice as the second largest.

```
class Solution {
public int dominantIndex(int[] nums) {
int maxnum=0, second=0, maxidx=0;
for (int i=0; i<nums.length; i++) {
if (nums[i] > maxnum) {
second = maxnum;
maxnum = nums[i];
maxidx = i;
}
else {
if (nums[i] > second) {
second = nums[i];
}
}
}
if (maxnum >= 2*second) {
return maxidx;
}
return -1;
}
}
```