bst postorder.java

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        if (root == null)
            return res;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode pre = null;
        stack.push(root);
        while(!stack.isEmpty()) {
            TreeNode curr = stack.peek();
            if (isLeaf(curr) || (pre != null && (pre == curr.left || pre == curr.right))) {
                res.add(stack.pop().val);
                pre = curr;
            }
            else {
                if (curr.right != null)
                    stack.push(curr.right);
                if (curr.left != null)
                    stack.push(curr.left);
            }
        }
        return res;
    }
    
    private boolean isLeaf(TreeNode node) {
        return node.left == null && node.right == null;
    }
}