Java solution 103 && 144

java/_103BinaryTreeZigzagLevelOrderTraversal.java

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+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+
+/**
+ * Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).
+ * <p>
+ * For example:
+ * Given binary tree [3,9,20,null,null,15,7],
+ *    3
+ *   / \
+ *  9  20
+ *  /  \
+ * 15   7
+ * return its zigzag level order traversal as:
+ * [
+ * [3],
+ * [20,9],
+ * [15,7]
+ * ]
+ * <p>
+ * Created by drfish on 6/7/2017.
+ */
+public class _103BinaryTreeZigzagLevelOrderTraversal {
+    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
+        List<List<Integer>> result = new ArrayList<>();
+        List<TreeNode> curr = new ArrayList<>();
+        boolean isOdd = true;
+
+        if (root == null) {
+            return result;
+        }
+        curr.add(root);
+
+        while (!curr.isEmpty()) {
+            List<TreeNode> next = new ArrayList<>();
+            List<Integer> level = new LinkedList<>();
+            for (TreeNode node : curr) {
+                if (node.left != null) {
+                    next.add(node.left);
+                }
+                if (node.right != null) {
+                    next.add(node.right);
+                }
+                if (isOdd) {
+                    level.add(node.val);
+                } else {
+                    level.add(0, node.val);
+                }
+            }
+            isOdd = !isOdd;
+            result.add(level);
+            curr = next;
+        }
+        return result;
+    }
+}

java/_144BinaryTreePreorderTraversal.java

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+import java.util.ArrayList;
+import java.util.List;
+import java.util.Stack;
+
+/**
+ * Given a binary tree, return the preorder traversal of its nodes' values.
+ * <p>
+ * For example:
+ * Given binary tree {1,#,2,3},
+ * 1
+ *  \
+ *   2
+ *  /
+ * 3
+ * return [1,2,3].
+ * <p>
+ * Note: Recursive solution is trivial, could you do it iteratively?
+ * Created by drfish on 6/8/2017.
+ */
+public class _144BinaryTreePreorderTraversal {
+    public List<Integer> preorderTraversal(TreeNode root) {
+        List<Integer> result = new ArrayList<>();
+        Stack<TreeNode> stack = new Stack<>();
+        while (root != null || !stack.isEmpty()) {
+            while (root != null) {
+                result.add(root.val);
+                stack.push(root);
+                root = root.left;
+            }
+            root = stack.pop();
+            root = root.right;
+        }
+        return result;
+    }
+}