133. Binary Tree Postorder Traversal

Postorder visits both subtrees before the node — useful when a node's result depends on its children (e.g. deleting a tree).

Left, Right, Node

dfs(node):
    if node is None: return
    dfs(node.left)
    dfs(node.right)
    visit node.val

Worked example

[1,null,2,3] → [3,2,1].

Complexity

• Time: O(n).
• Space: O(h).

import java.util.ArrayList;
import java.util.List;

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        dfs(root, res);
        return res;
    }

    private void dfs(TreeNode node, List<Integer> res) {
        if (node == null) return;
        dfs(node.left, res);
        dfs(node.right, res);
        res.add(node.val);
    }
}