CODE 142: Binary Tree Postorder Traversal

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

For example:

Given binary tree 

{1,#,2,3}

,

1
\
2
/
3

return 

[3,2,1]

.

public ArrayList<Integer> postorderTraversal(TreeNode root) {
// IMPORTANT: Please reset any member data you declared, as
// the same Solution instance will be reused for each test case.
if (null == root) {
return new ArrayList<Integer>();
}
ArrayList<Integer> result = new ArrayList<Integer>();
Stack<TreeNode> stack = new Stack<TreeNode>();
while (!stack.isEmpty() || null != root) {
if (null != root) {
stack.push(root);
root = root.left;
} else {
root = stack.peek();
if (null != root.right) {
TreeNode right = root.right;
stack.peek().left = null;
stack.peek().right = null;
stack.push(right);
root = right.left;
} else {
stack.pop();
result.add(root.val);
root = null;
}
}
}
return result;
}