Find the dominant element of nums by using a hashmap to maintain element frequency, we denote the dominant element as x and its frequency as f.

For each index in [0, n - 2], calculate f1, x’s frequency in the subarray [0, i] when looping the index. And f2, x’s frequency in the subarray [i + 1, n - 1] which is equal to f - f1. Then we can check whether x is dominant in both subarrays.