binary tree traverse

Author: Narahari-Sundaragopalan

exercises/leetcode/inorderTraversal.js

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+/**
+ * Given a binary tree, return the inorder traversal of its nodes' values.
+
+Example:
+
+Input: [1,null,2,3]
+   1
+    \
+     2
+    /
+   3
+
+Output: [1,3,2]
+Follow up: Recursive solution is trivial, could you do it iteratively?
+ */
+const traverse = (node, result) => {
+    if (node !== null) {
+        if (node.left !== null) {
+            traverse(node.left, result);
+        }
+        result.push(node.val);
+        if (node.right !== null) {
+            traverse(node.right, result);
+        }
+    }
+}
+const inOrderTraversal = root => {
+    const result = [];
+
+    traverse(root, result);
+
+    return result;
+}
+
+// Iterative solution
+const inorderTraversal = root => {
+    const result = [];
+    const stack = [];
+    let curr = root;
+    
+    while (curr !== null || stack.length()) {
+        while (curr !== null) {
+            stack.push(curr);
+            curr = curr.left;
+        }
+        curr = stack.pop();
+        result.push(curr.val);
+        curr = curr.right;
+    }
+
+    return result;
+}

exercises/leetcode/zigzagTraversal.js

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+/**
+ * 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).
+
+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]
+]
+ */
+
+const traverseZigzag = (node, level, output) => {
+    if (!node) {
+        return output;
+    }
+
+    if (output.length === level) {
+        output.push([]);
+    }
+
+    if (level % 2 !== 0) {
+        output[level].unshift(node.val);
+    } else {
+        output[level].push(node.val);
+    }
+
+    traverseZigzag(node.left, level + 1, output);
+    traverseZigzag(node.right, level + 1, output);
+}
+
+const zigzagLevelOrder = root => {
+    const output = [];
+
+    traverseZigzag(root, 0, output);
+
+    return output;
+}