tree basics

Author: idf

src/BinaryTreePostorderTraversal/Solution.java

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+package BinaryTreePostorderTraversal;
+
+import commons.datastructures.TreeNode;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Stack;
+
+/**
+ * User: Danyang
+ * Date: 1/27/2015
+ * Time: 10:38
+ *
+ * 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].
+
+ Note: Recursive solution is trivial, could you do it iteratively?
+ */
+public class Solution {
+    /**
+     * Post order L R c
+     * @param root
+     * @return
+     */
+    public List<Integer> postorderTraversal(TreeNode root) {
+        List<Integer> ret = new ArrayList<>();
+        Stack<TreeNode> stk = new Stack<>();
+        if(root==null)
+            return ret;
+        stk.add(root);
+        while(!stk.empty()) {
+            TreeNode cur = stk.pop();
+            ret.add(0, cur.val);
+            if(cur.left!=null)
+                stk.add(cur.left);
+            if(cur.right!=null)
+                stk.add(cur.right);
+        }
+        return ret;
+    }
+}

src/BinaryTreePreorderTraversal/Solution.java

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+package BinaryTreePreorderTraversal;
+
+import commons.datastructures.TreeNode;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Stack;
+
+/**
+ * User: Danyang
+ * Date: 1/27/2015
+ * Time: 10:34
+ * Given a binary tree, return the preorder traversal of its nodes' values.
+
+ For example:
+ Given binary tree {1,#,2,3},
+ 1
+ \
+  2
+ /
+ 3
+ return [1,2,3].
+
+ Note: Recursive solution is trivial, could you do it iteratively?
+ */
+public class Solution {
+    /**
+     * Pre-order: c L R
+     * @param root
+     * @return
+     */
+    public List<Integer> preorderTraversal(TreeNode root) {
+        Stack<TreeNode> stk = new Stack<>();
+        List<Integer> ret = new ArrayList<>();
+        if(root==null)
+            return ret;
+
+        stk.add(root);
+        while(!stk.empty()) {
+            TreeNode cur = stk.pop();
+            ret.add(cur.val);
+            if(cur.right!=null)
+                stk.add(cur.right);
+            if(cur.left!=null)
+                stk.add(cur.left);
+        }
+        return ret;
+    }
+}