[145. Binary Tree Postorder Traversal] #leetcode

145_0822.java

class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        LinkedList<Integer> result = new LinkedList<Integer>();
        
        TreeNode curNode = root;
        Deque<TreeNode> stack = new ArrayDeque<TreeNode>();
        while ( !stack.isEmpty() || curNode != null ) {
            if (curNode != null) {
                stack.push(curNode);
                result.addFirst(curNode.val);
                curNode = curNode.right;
            } else {
                curNode = stack.pop().left;
            }
        }
        
        return result; 
    }
}

class Solution1 {
    public List<Integer> postorderTraversal(TreeNode root) {
        LinkedList<Integer> result = new LinkedList<Integer>();
        if (root == null) return result;
        
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while ( !stack.isEmpty() ) {
            TreeNode curNode = stack.pop();
            result.addFirst(curNode.val);
            if (curNode.left != null) stack.push(curNode.left);
            if (curNode.right != null) stack.push(curNode.right);
            
        }
        return result; 
    }
}