From cc25a312679da224c703165a0ce8ffa286e17ec8 Mon Sep 17 00:00:00 2001
From: Krishna Gupta <krishnaguptakp21@gmail.com.com>
Date: Thu, 30 Oct 2025 20:17:06 +0530
Subject: [PATCH] Add pre-order traversal

---
 DSA/pre-order traversal.cpp | 122 ++++++++++++++++++++++++++++++++++++
 1 file changed, 122 insertions(+)
 create mode 100644 DSA/pre-order traversal.cpp

diff --git a/DSA/pre-order traversal.cpp b/DSA/pre-order traversal.cpp
new file mode 100644
index 0000000..a1f6490
--- /dev/null
+++ b/DSA/pre-order traversal.cpp	
@@ -0,0 +1,122 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+/**
+ * @brief Structure for a binary tree node.
+ */
+struct TreeNode {
+    int val;
+    TreeNode* left;
+    TreeNode* right;
+    TreeNode(int value) : val(value), left(nullptr), right(nullptr) {}
+};
+
+/**
+ * @brief Builds a binary tree level-wise from user input.
+ * 
+ * Input format: integers separated by spaces, -1 for null.
+ * Example: 1 2 3 4 5 -1 6 -1 -1 -1 -1 -1 -1
+ * 
+ * @return Pointer to the root of the tree.
+ */
+TreeNode* buildTreeLevelOrder() {
+    cout << "Enter nodes in level order (-1 for NULL):\n";
+    string line;
+    getline(cin, line);
+    if (line.empty()) getline(cin, line);  // handle leftover newline
+
+    stringstream ss(line);
+    vector<int> values;
+    int x;
+    while (ss >> x) values.push_back(x);
+
+    if (values.empty() || values[0] == -1) return nullptr;
+
+    TreeNode* root = new TreeNode(values[0]);
+    queue<TreeNode*> q;
+    q.push(root);
+    int i = 1;
+
+    while (!q.empty() && i < (int)values.size()) {
+        TreeNode* node = q.front();
+        q.pop();
+
+        // Left child
+        if (i < values.size() && values[i] != -1) {
+            node->left = new TreeNode(values[i]);
+            q.push(node->left);
+        }
+        i++;
+
+        // Right child
+        if (i < values.size() && values[i] != -1) {
+            node->right = new TreeNode(values[i]);
+            q.push(node->right);
+        }
+        i++;
+    }
+
+    return root;
+}
+
+/**
+ * @brief Recursive preorder traversal.
+ */
+void preorderRecursive(TreeNode* root, vector<int> &result) {
+    if (!root) return;
+    result.push_back(root->val);
+    preorderRecursive(root->left, result);
+    preorderRecursive(root->right, result);
+}
+
+/**
+ * @brief Iterative preorder traversal using stack.
+ */
+vector<int> preorderIterative(TreeNode* root) {
+    vector<int> result;
+    if (!root) return result;
+
+    stack<TreeNode*> st;
+    st.push(root);
+
+    while (!st.empty()) {
+        TreeNode* node = st.top();
+        st.pop();
+        result.push_back(node->val);
+
+        if (node->right) st.push(node->right);
+        if (node->left) st.push(node->left);
+    }
+    return result;
+}
+
+/**
+ * @brief Utility function to print traversal.
+ */
+void printTraversal(const vector<int> &res) {
+    for (int val : res) cout << val << " ";
+    cout << "\n";
+}
+
+int main() {
+    ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    TreeNode* root = buildTreeLevelOrder();
+
+    if (!root) {
+        cout << "Tree is empty.\n";
+        return 0;
+    }
+
+    vector<int> preorderRes;
+    preorderRecursive(root, preorderRes);
+    cout << "\nPreorder (Recursive): ";
+    printTraversal(preorderRes);
+
+    vector<int> iterativeRes = preorderIterative(root);
+    cout << "Preorder (Iterative): ";
+    printTraversal(iterativeRes);
+
+    return 0;
+}