652. Find Duplicate Subtrees

Based upon the Post order traversal AND serialize deserialize, HOW?

subtree always involves leaves therefore evolving from leaf seems better idea.
In post order priority is given to left, right then to leaf.

• make post order traversal first.
• after done traversing comes the calculation part.
• maintain the count of every subtree from leaf to root and store them in map.
• if occurrence = 2 store in the answer, that value is the final, it wont be updating again.
• because we only moving upward and always adding something.

class Solution {
    HashMap<String, Integer> map;
    ArrayList<TreeNode> arr;
    
    public String pre(TreeNode root) {
        if (root == null) 
            return "#";
        
        String left = pre(root.left);
        String right = pre(root.right);
        
        String curr = left + "," + right + "," + Integer.toString(root.val);
        map.put(curr, map.getOrDefault(curr, 0) + 1);
        if (map.get(curr) == 2) 
            arr.add(root);
        
        return curr;
    }
    
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        map = new HashMap<>();
        arr = new ArrayList<>();
        String ans = pre(root);
        return arr;
    }
}