Added Zigzag Traversal of Binary Tree in C++

C_CPP/DSA/Binary Tree/Zigzag_Traversal_Binary_Tree.md

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+# Zigzag Traversal of Binary Tree
+
+## Overview
+
+Zigzag traversal (also called **spiral traversal**) is a level-order traversal technique for a binary tree where the direction of traversal alternates at each level.
+
+* Level 0: Left to Right
+* Level 1: Right to Left
+* Level 2: Left to Right
+* and so on...
+
+This traversal is useful when a non-linear, alternating view of tree levels is required.
+
+---
+
+## Example
+
+Consider the following binary tree:
+
+```
+        1
+       / \
+      2   3
+     / \ / \
+    4  5 6  7
+```
+
+**Zigzag Traversal Output:**
+
+```
+1 3 2 4 5 6 7
+```
+
+Explanation:
+
+* Level 0 → `1` (Left to Right)
+* Level 1 → `3 2` (Right to Left)
+* Level 2 → `4 5 6 7` (Left to Right)
+
+---
+
+## Algorithm
+
+1. Use a **queue** to perform level-order traversal.
+2. Maintain a boolean flag `leftToRight` to track traversal direction.
+3. For each level:
+
+   * Store node values in a temporary vector.
+   * Reverse the vector if `leftToRight` is `false`.
+4. Append the values to the result.
+5. Toggle the `leftToRight` flag after each level.
+
+---
+
+## C++ Implementation
+
+```cpp
+#include <iostream>
+#include <queue>
+#include <vector>
+using namespace std;
+
+struct Node {
+    int data;
+    Node* left;
+    Node* right;
+
+    Node(int val) {
+        data = val;
+        left = right = NULL;
+    }
+};
+
+vector<int> zigzagTraversal(Node* root) {
+    vector<int> result;
+    if (root == NULL)
+        return result;
+
+    queue<Node*> q;
+    q.push(root);
+    bool leftToRight = true;
+
+    while (!q.empty()) {
+        int size = q.size();
+        vector<int> level(size);
+
+        for (int i = 0; i < size; i++) {
+            Node* temp = q.front();
+            q.pop();
+
+            int index = leftToRight ? i : size - i - 1;
+            level[index] = temp->data;
+
+            if (temp->left)
+                q.push(temp->left);
+            if (temp->right)
+                q.push(temp->right);
+        }
+
+        for (int val : level)
+            result.push_back(val);
+
+        leftToRight = !leftToRight;
+    }
+
+    return result;
+}
+
+int main() {
+    Node* root = new Node(1);
+    root->left = new Node(2);
+    root->right = new Node(3);
+    root->left->left = new Node(4);
+    root->left->right = new Node(5);
+    root->right->left = new Node(6);
+    root->right->right = new Node(7);
+
+    vector<int> output = zigzagTraversal(root);
+
+    for (int val : output)
+        cout << val << " ";
+
+    return 0;
+}
+```
+
+---
+
+## Time and Space Complexity
+
+* **Time Complexity:** `O(n)` — each node is visited once.
+* **Space Complexity:** `O(n)` — queue and result storage.
+
+---
+
+## Notes
+
+* Zigzag traversal is applicable to **any binary tree**, not limited to Binary Search Trees.
+* This approach avoids explicit vector reversal by calculating indices based on direction.
+
+---
+
+## References
+
+* Level Order Traversal
+* Binary Tree Basics