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].

 

/**
 * Definition for a binary tree node.
 * 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<>();  
        if (root != null) {
        	Stack<TreeNode> stack = new Stack<TreeNode>();  
            stack.add(root);  
            TreeNode c = null;
            while (!stack.isEmpty()) {
            	c = stack.peek();
            	if (c.left != null && root != c.left && root != c.right) {
            		stack.push(c.left);
            	} else if (c.right != null && root != c.right) {
            		stack.push(c.right);
            	} else {
            		res.add(stack.pop().val);
            		root = c;
            	}
            }
        }
        return res;
    }
}