refactor 145

Author: fishercoder1534

src/main/java/com/fishercoder/solutions/_145.java

@@ -4,6 +4,7 @@
 
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.LinkedList;
 import java.util.List;
 import java.util.Stack;
 
@@ -30,7 +31,7 @@ public static class Solution1 {
      * so that it becomes root->right->left,
      * and then reverse the result to get left->right->root.
      */
-    public static List<Integer> postorderTraversal(TreeNode root) {
+    public List<Integer> postorderTraversal(TreeNode root) {
       List<Integer> result = new ArrayList();
       if (root == null) {
         return result;
@@ -53,6 +54,35 @@ public static List<Integer> postorderTraversal(TreeNode root) {
   }
 
   public static class Solution2 {
+    /**Or use a LinkedList and add values to the head, then no reverse is needed.
+     * the linked list contents get added like this:
+     *
+     * root
+     * right, root
+     * left, right, root
+     * */
+    public List<Integer> postorderTraversal(TreeNode root) {
+      List<Integer> list = new LinkedList<>();
+      if (root == null) {
+        return list;
+      }
+      Stack<TreeNode> stack = new Stack<>();
+      stack.push(root);
+      while (!stack.isEmpty()) {
+        TreeNode curr = stack.pop();
+        list.add(0, curr.val);
+        if (curr.left != null) {
+          stack.push(curr.left);
+        }
+        if (curr.right != null) {
+          stack.push(curr.right);
+        }
+      }
+      return list;
+    }
+  }
+
+  public static class Solution3 {
     public List<Integer> postorderTraversal(TreeNode root) {
       List<Integer> result = new ArrayList();
       return post(root, result);

src/test/java/com/fishercoder/_145Test.java

@@ -16,13 +16,15 @@
 public class _145Test {
     private static _145.Solution1 solution1;
     private static _145.Solution2 solution2;
+    private static _145.Solution3 solution3;
     private static TreeNode root;
     private static List<Integer> expected;
 
     @BeforeClass
     public static void setup() {
         solution1 = new _145.Solution1();
         solution2 = new _145.Solution2();
+        solution3 = new _145.Solution3();
     }
 
     @Test
@@ -31,8 +33,10 @@ public void test1() {
         TreeUtils.printBinaryTree(root);
         CommonUtils.printList(solution1.postorderTraversal(root));
         CommonUtils.printList(solution2.postorderTraversal(root));
+        CommonUtils.printList(solution3.postorderTraversal(root));
         expected = new ArrayList<>(Arrays.asList(8, 9, 7, 4, 2, 5, 6, 3, 1));
         assertEquals(expected, solution1.postorderTraversal(root));
         assertEquals(expected, solution2.postorderTraversal(root));
+        assertEquals(expected, solution3.postorderTraversal(root));
     }
 }