add

Author: zhaoguanchen

src/leetcode/solution/tree/traversal/BinaryTreePostorderTraversal.java

@@ -38,22 +38,24 @@ private static void helper(TreeNode root, List<Integer> result) {
     public static List<Integer> postorderIterator(TreeNode root) {
         LinkedList<Integer> ans = new LinkedList<>();
         Stack<TreeNode> stack = new Stack<>();
-        if (root == null) {
-            return ans;
-        }
+        stack.add(root);
 
-        stack.push(root);
         while (!stack.isEmpty()) {
-            TreeNode cur = stack.pop();
-            ans.addFirst(cur.val);
-            if (cur.left != null) {
-                stack.push(cur.left);
+            TreeNode node = stack.pop();
+            ans.addFirst(node.val);
+
+            if (node.left != null) {
+                stack.add(node.left);
             }
-            if (cur.right != null) {
-                stack.push(cur.right);
+
+            if (node.right != null) {
+                stack.add(node.right);
             }
         }
+
         return ans;
+
+
     }
 
 

src/leetcode/solution/tree/traversal/BinaryTreePreorderTraversal.java

@@ -38,32 +38,27 @@ private static void helper(TreeNode root, List<Integer> result) {
     }
 
     public static List<Integer> preorderIterator(TreeNode root) {
-        if (root == null) {
-            return new ArrayList<>();
-        }
-
-        List<Integer> res = new ArrayList<>();
+        List<Integer> ans = new ArrayList<>();
         Stack<TreeNode> stack = new Stack<>();
-
         stack.add(root);
 
         while (!stack.isEmpty()) {
-            TreeNode current = stack.pop();
-            if (current == null) {
-                break;
-            }
-
-            res.add(current.val);
+            TreeNode node = stack.pop();
+            ans.add(node.val);
 
-            if (current.right != null) {
-                stack.add(current.right);
-            }
-            if (current.left != null) {
-                stack.add(current.left);
+            if (node.right != null) {
+                stack.add(node.right);
             }
 
+            if (node.left != null) {
+                stack.add(node.left);
+            }
         }
-        return res;
+
+        return ans;
+
 
     }
+
+
 }