MEDIUM/src/medium/BinaryTreePostOrderTraversal.java

fishercoder1534 · fishercoder1534 · commit 5445b27fe4b0 · 2016-08-27T13:14:54.000-07:00
diff --git a/MEDIUM/src/medium/BinaryTreePostOrderTraversal.java b/MEDIUM/src/medium/BinaryTreePostOrderTraversal.java
@@ -0,0 +1,57 @@
+package medium;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Stack;
+
+import utils.CommonUtils;
+import classes.TreeNode;
+
+public class BinaryTreePostOrderTraversal {
+        /**I was really confused about how iterative version works for POST-order traversal, since we'lld need to add a field in TreeNode
+        class called "visited", to mark this node has been visited before, otherwise it goes into indefinite loop.
+        Then I turned to Discuss, only found that the top-voted one is actually using such a trick:
+        modify the code for pre-order traversal so that it becomes root->right->left, and then reverse the result to get left->right->root, so tricky!!!*/
+    public static List<Integer> postorderTraversal_iterative(TreeNode root) {
+        List<Integer> result = new ArrayList();
+        Stack<TreeNode> stack = new Stack();
+        if(root == null) return result;
+        stack.push(root);
+        while(!stack.isEmpty()){
+            root = stack.pop();
+            result.add(root.val);
+            if(root.left != null) stack.push(root.left);
+            if(root.right != null) stack.push(root.right);
+        }
+        Collections.reverse(result);
+        return result;
+    }
+    
+    public static void main(String...strings){
+//        TreeNode root = new TreeNode(1);
+//        root.left = new TreeNode(2);
+//        root.right = new TreeNode(3);
+        
+//        TreeNode root = new TreeNode(1);
+//        root.left = new TreeNode(2);
+        
+        TreeNode root = new TreeNode(1);
+        root.right = new TreeNode(2);
+        List<Integer> result = postorderTraversal_iterative(root);
+        CommonUtils.printList(result);
+    }
+    
+    public List<Integer> postorderTraversal_recursive(TreeNode root) {
+        List<Integer> result = new ArrayList();
+        return post(root, result);
+    }
+    
+    List<Integer> post(TreeNode root, List<Integer> result){
+        if(root == null) return result;
+        post(root.left, result);
+        post(root.right, result);
+        result.add(root.val);
+        return result;
+    }
+}
