LeetCode: Binary Tree PostOrder Traversal

 /*  
 ** This is problem can be solved in a very smart way.  
 ** The key point here is if you use a stack to do DFS  
 ** (Left-to-right), the order of visiting is preorder.  
 ** But, if you do in the opposite way, it will generate  
 ** the opposite result of postorder traversal.  
 **  
 ** Preorder, stack push right first, then left  
 ** PostOrder, stack push left first, then right, after  
 ** finish the loop, reverse the result.  
 */  
 public class Solution {  
   public List<Integer> postorderTraversal(TreeNode root) {  
     List<Integer> ans = new ArrayList<Integer>();  
     if (root == null) return ans;  
     Stack<TreeNode> stack = new Stack<TreeNode>();  
     stack.push(root);  
     while (!stack.empty()) {  
       TreeNode curr = stack.pop();  
       ans.add(curr.val);  
       if (curr.left != null)   
         stack.push(curr.left);  
       if (curr.right != null)  
         stack.push(curr.right);  
     }  
     Collections.reverse(ans);  
     return ans;  
   }  
 }