add tree traversal algorithms

Author: betomoedano

binary-trees/traversals.js

@@ -0,0 +1,38 @@
+/**
+ * @param {TreeNode} root
+ * @return {number[]}
+ * This algorithm works by iterating through the tree "in-place",
+ * without using any additional stack or queue data structures.
+ * This is what allows it to have a space complexity of O(1).
+ * Time O(n)
+ * Space O(1)
+ * Preorder Traversal
+ */
+var preorderTraversal = function (root) {
+  let current = root;
+  let result = [];
+
+  while (current) {
+    if (!current.left) {
+      result.push(current.val);
+      current = current.right;
+    } else {
+      let predecessor = current.left;
+
+      while (predecessor.right && predecessor.right !== current) {
+        predecessor = predecessor.right;
+      }
+
+      if (!predecessor.right) {
+        result.push(current.val);
+        predecessor.right = current;
+        current = current.left;
+      } else {
+        predecessor.right = null;
+        current = current.right;
+      }
+    }
+  }
+
+  return result;
+};