Binary Tree Postorder Traversal

Given the root of a binary tree, return the postorder traversal of its nodes' values.

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Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution {
    public IList<int> PostorderTraversal(TreeNode root) {
        var result = new List<int>();
        if (root == null) return result;

        var stack = new Stack<TreeNode>();
        stack.Push(root);

        while (stack.Count > 0) {
            var node = stack.Pop();
            result.Insert(0, node.val);

            if (node.left != null) stack.Push(node.left);
            if (node.right != null) stack.Push(node.right);
        }

        return result;
        
    }
}