Given the root of a binary tree, return the most frequent subtree sum.
If there is a tie, return all the values with the highest frequency in any order.
The subtree sum of a node is defined as the sum of all the node values formed by the subtree rooted at that node (including the node itself).
This problem requires computing the sum of every subtree in a binary tree and then determining which sum appears most frequently. A natural approach is to perform a postorder traversal, since subtree sums depend on left and right child results. While computing each subtree’s total, you can store the frequency of each sum in a hash map. After the traversal finishes, identify the sum(s) with the highest frequency. The main idea is combining DFS traversal with frequency counting.
